Nobility, and with it the Feudal System, had largely died out on Terra by the start of the 21st century, replaced in most cases by more equitable forms of government. As humanity expanded out into the stars though, the slow pace and sheer expense of space travel and communication made it increasingly impractical for these systems of governments to function on an interstellar scale. In the middle of the 24th century, Michael Cameron, first elected ruler of the Terran Hegemony, revived the system of nobility as a means of exerting authority over and ensuring the loyalty of even the most far-flung regions of the Hegemony. Director-General Cameron was able to overcome resistance to this medieval revival by making the initial distribution of titles impartial - rewarding gifted artists and scientists as much as industrialists and military leaders - as well as non-discriminatory and non-hereditary. Many so ennobled went on to accomplish great works for the Hegemony, helping justify the system's revitalization.
After its success within the Terran Hegemony, nobility spread across the Inner Sphere, resulting in the creation of the six (later five) Great Houses and many minor houses of nobility. Many titles of nobility eventually became hereditary, by custom if not by law, and within each of the Successor States variations on the initial hierarchy of nobles developed along cultural lines. By the 31st century nobility had become a fact of life for the Inner Sphere, providing a sense of identity and political stability over the centuries of warfare.
- Knight (former title)
- Earl (former title)
- Duke 
Nobility in the Federated Suns grew out of the realization that, with the size of their interstellar nation, traditional democratic principles were too slow in reacting to changing situations. Reasoning that local leaders needed greater discretionary powers to act on their own initiative, Simon Davion divided the realm into five Principalities, each headed by a Prince who in turn appointed nobles beneath them. Most titles of nobility went to individuals who had already been elected to positions of leadership by the populace, and the titles were only temporary at first; it would not be until much later that many became hereditary, often owing to the good qualities of a particular noble and the hope their offspring would be equally capable. Although the Davion Civil War ended the Era of the Five Princes, the nobility they created remained to continue governing the Federate Suns.
While nobles could and did abuse their position, the populace had means by which to redress grievances. In a Public Appeal, the people petition the target noble's superiors to have them removed and either replaced by the next in line or, if the entire noble family was seen as unworthy, strip them all of their rank. A noble also had the right to remove a subordinate on their own volition as well, although in both cases they needed the approval of their superior (or of the First Prince when dealing with removing an entire family) and it could take years before a final decision was rendered. To guarantee the right of the public to a Public Appeal, the Laws of Noble Conduct and Review were signed in 2634 which allowed the state to take action against nobles who attempted to squash such criticism, up to and including military action if necessary.
Direct action was another method the people took when abused by the nobility, particularly local nobility, ranging from non-violent demonstrations to armed insurrection. Labor strikes in particular were most effective against planetary nobles, as the economic disruption often caused their fellows and superiors to apply pressure in favor of resolving the situation as quickly as possible. Violence was an extreme measure of last resort for a planet's population, and in such cases liable to a response under the Planetary War Powers Act. With the authority of this Act, the First Prince could occupy a planet in revolt with military forces and replace the planetary government with a Military Governor. Whilst the military and DMI dealt with the insurrectionists, federal judges examined the actions of the relevant nobility and punished them harshly if found guilty of crimes against the people, all the while a new government was set up to take over once the crisis was resolved.
- Knight was the lowest class of nobility within the Federated Suns. Knighthood could be bestowed on anyone by any noble ruler; likewise it could be revoked just as easily, though only by one of equal or greater stature (i.e. a knighthood created by a Duke could only be undone by another Duke or the First Prince). As such, knighthood was a popular reward to bestow upon a citizen who went about and beyond the call of duty. Along with their title, Knights were given ownership of a land grant, although they did not have any other powers associated with the nobility.
- Baron/Baroness was the next highest class of nobility and the lowest one with actual political authority. Barons typically ruled over a city, a large area of land, or a particular industrial center; in rare cases they might instead control a particularly important company rather than a tract of land, although such Barons were derisively known as "Lyran Lords" or "second-rate Barons." Appointed by a Count, Barons were either the elected head of a local government or worked alongside it, collecting taxes and controlling access to natural resources. Barons and Baronesses were the nobles most responsible to the people over whom they ruled, and the ones most subject to dismissal in the case of public anger.
- Count/Countess was the title of nobility given to rulers of particularly important cities, planetary continents, moons, or even sparsely-populated worlds. Appointed by a Duke (with the approval of the federal government), a Count had immense power within their area of responsibility: how local governments operated, the regulation of business transactions, allocation of resources, and the enactment of policies created by their Duke and First Prince. Except on certain planets with specific legal limitations, Counts were not answerable to local government authority and could override the actions of civil governments and nobles below them, with military force if necessary.
- Marquess/Marquesas was a title recently revived at the beginning of the 31st century, technically above the rank of Count but below Duke, and used one of two ways. Originally the title was used for rulers of frontier worlds with little economic development or otherwise not worthy of a ducal designation. Given to lesser nobility or even commoners as a reward, the title was only temporary, usually for a period of ten to fifteen years. If the ruler proved competent and helped develop the planet their position was made permanent. First Prince Ian Davion and his brother Hanse were instrumental in reviving the title to develop worlds along the Periphery border. Alternatively, in duchies with multiple planets, a Duke or Duchess may use the title to bestow control of one of their worlds to a son or daughter, particularly if they are not in line to succeed them.
- Duke/Duchess was the highest level of nobility, directly beneath the First Prince. A Duke's authority ranged from a single world or several worlds within a solar system to multiple solar systems spread out over several light-years. A Duke or Duchess had great authority over the administration of their duchy and their decisions could have immediate and lasting impact. In addition to their political power, ducal families also tended to be the wealthiest in the Federated Suns. Not all Dukes were equal though: traditionally the three most senior ducal families each exerted control over one of the three Marches which made up the Federated Suns, with lesser duchies beneath them.
Archon Katherine Steiner instituted nobility within the Lyran Commonwealth to reward her political allies and place them in control of key worlds within the Commonwealth. Inspired by the stories she heard growing up and as a means of providing a shared cultural basis for the disparate realm, Katherine based this new nobility on ancient Germanic forms, including the use of the nobiliary particle von. Many titles of nobility were hereditary and included various political, financial and landholding privileges. However, the ability for nobles to influence the government was limited: most could not serve in the Estates General, and few were notable enough to be invited to join the Commonwealth Council.
Nobility within the Lyran state was also distinct for being more "open" than in other realms: it was much easier for so-called commoners to be ennobled and become a member of the aristocracy, especially for industrialists and others who contribute to the Lyran economy. This often resulted in conflict between landholding and corporate aristocrats, particularly when the latter's assets reside within the former's territory. As often as not, legal issues were decided in the corporate noble's favor, with courts seeing more value in who does the work over where it happens to be located.
- Knight was the lowest class of nobility in the Lyran state. Formally addressed as a "Knight of the Commonwealth" (and later "Knight of the Alliance"), the title was bestowed upon individuals who earned certain military or achievement awards such as the Medal of Honor or the Golden Fist. The title was not hereditary and rarely carried with it political authority or other privileges, but it confered respect and honor throughout Lyran society. Knights were also allowed to serve in the Estates General.
- Baron/Baroness was the lowest order of hereditary nobility and the highest allowed to serve in the Estates General. Originating from the Archon or the local Duke, the title of Baron was usually granted as a reward for distinguished achievements; certain awards such as the Dealby Prize for the Advancement of Science came with this title. Along with a generous monthly stipend, baronies included a land grant, anywhere from a small city to an entire world depending on the individual and other considerations.
- Graf was a hereditary title given to landholders of large territories, such as an island or continent, or the owners of large industries. A Grafship could be granted by either the Archon or the local Duke; the heir apparent of a duchy was also traditionally given the title of Graf. Oftentimes the Archon would appoint a Graf as part of a political move, leading to antagonism between the local nobility and the populace, or to act as their representative on worlds where they were unpopular. Exceptionally powerful Grafs were given the title Landgrave.
- Margrave was a non-hereditary title given to the commander of one of the military districts along the Lyran state's borders. A Margrave had full executive authority over all worlds which fell under their control, although only rarely could they wield their power in non-military matters. Typically when a Margrave moved to another post or retired, their title was transferred to their replacement. As reward for exceptional services to the state, some officers could retain the title, and their children were given baronies and could attend one of the military academies.
- Duke/Duchess was the highest title of nobility, awarded only by the Archon to those who had (or were about to acquire) a controlling interest in one or more worlds or major interstellar industries. A Duke or Duchess' status among their peers was determined by the wealth of the worlds or industries that they control. Particularly influential individuals could be awarded the title of Grand Duke, although the title itself didn't confer any additional privileges.
Nobility in the Draconis Combine are charged with running the government and presenting "the face of House Kurita to the lesser worlds of the Inner Sphere." From the highest Warlords and ministry heads to the lowest landholders, all are charged with implementing the will of the Coordinator. As the Coordinator can overrule any decision, most nobles are circumspect until they are sure they have their ruler's approval, and intrigue is common at the Imperial Court on Luthien as each vies for supremacy over the other. As thanks for the burden of serving the Coordinator and looking out for the welfare of the lower classes, known as the "Suffering of the Lion", Kuritan nobility take advantage of the many intellectual and luxurious pleasures denied to their inferiors. They may, for example, collect literature and media from the other Successor States, and may travel to other realms for both professional and personal reasons (though such trips are often under surveillance by the Internal Security Force).
Kuritan nobility uses distinct titles roughly analogous to those of House Cameron's Peer List. In general the proper Kurita/Japanese titles are used in most ceremonial and formal occasions, while the Cameron usage is used in more common situations. Some of the Japanese variations only came into vogue following a change enacted by then-Coordinator Theodore Kurita in 3057 to assuage the wounded egos of those nobles who resented his ongoing reforms, by 3067 use Daimyo and Shogun had almost totally supplanted their more common variations. For centuries, the male form of the title was used regardless of the holder's actual gender, though that began to change in the latter part of the thirty-first century.
- Lord/Lady (Knight/Lady) - If the title holder is a MechWarrior Samurai is the proper term  
- Shugo (Baron/Baroness)  
- Tozama Daimyo (Count/Countess)  
- Daimyo (Marquis/Marquessa)  
- Shogun (Duke/Duchess)  
The nobility of the Draconis Combine, more than any other, practice the art of the vendetta to settle disputes of honor. Compared to simple political assassinations, vendettas are performed under strict rules - some laid out in The Assassin's Handbook but many unwritten - to settle old accounts and make sure any blood imbalances are corrected. Vendettas must be registered with House Kurita and are subject to the Coordinator's approval; the Coordinator can also settle the vendetta by decree if they deem it necessary. Women cannot be the subject to a vendetta - if a woman is slighted by another, she has no right or obligation of revenge, while a man slighting a woman is an offense to her husband/father/etc. - though an easing of attitudes in the thirty-first century has allowed women serving in the military to seek recompense. Vendettas cannot exist between the nobility and lower classes (though the middle class, in an attempt to ape their betters, often practice "plastic vendettas" amongst themselves), and given the clear superiority of Kuritan nobility to all others, formal vendettas may only exist against nobles in other realms if they are a rank higher than the offended Kuritan noble. Vendettas must also be carried out by family members of the offended party, though that does not stop the practice of "adopting" honorary family members to carry out the blood price.
Free Worlds League
As in the other Successor States, titles of nobility came to pass within the Free Worlds League. However, many of these were purely honorary, and as befitted the fractious nature of the League there was little consistency among the different provinces: nominally a planetary ruler was styled Duke while lesser nobles were Barons, but in truth each planet had their own naming traditions to go along with their unique form of government. On Shiloh, the head of the theocratic government was known as the "Blessed Leader" (though much to their annoyance they were typically styled "Lord/Lady" elsewhere in the League), while in the Principality of Gibson a "Principal" ruled for a seven-year term (after which they were known as "Emeritus" and formed an advisory board for the new Principal). The titles varied in their grandiosity as much as their actual scope of power, from "Landholder" and "Exalt" to "Prince" and "Emperor".
For the leaders of the major provinces of the League, "Duke" or "Duchess" was the most common title and had preeminence over all other nobles within their territory. Female leaders of the Duchy of Andurien were styled Dame while Grand Duke was used for the ruler of the Duchy of Oriente. Below Duke was Earl and Count, who held power over smaller provinces or individual worlds, such as in the Stewart Commonality or on Tamarind. Even here though there were exceptions: the rulers of the Principality of Regulus, while of the same rank as other major duchies, were traditionally styled "Prince" until the late 27th century, after which they assumed the title "Count of Harmony," only to again use the title "Prince" during the latter half of the 31st century.
Nobility within the Capellan Confederation is divided into two types, the Sheng and the Barduc. The Sheng are hereditary nobles, most of whom who trace their ancestry to the founding states of the Confederation, while others were rewarded their entitlement afterwards by the Chancellor. The Barduc are military commanders who have been ennobled, ideally as reward for their service although in some cases simply for making a sufficient monetary contribution. Barduc titles are typically not hereditary, although they can be at the Chancellor or nobleman's discretion.
There are five ranks of nobility in the Confederation, mirroring the Peer List created by House Cameron, used by both the Sheng and Barduc. After the onset of the Succession Wars, some titles took on a Liaoist theme to differentiate them from other realms:
- Lord/Lady (Knight/Lady)
- Mandrinn/Mandrissa (Baron/Baroness)
- Shonso/Shanna (Count/Countess)
- Lama/Lamia (Marquis/Marquessa)
Capellan nobility enjoy a number of privileges thanks to their rank. They may levy taxes on populations living under their direct control; sell junior officer commissions (to the rank of Subcommander) in the Capellan Confederation Armed Forces; act as judge and jury in criminal and civil cases occurring within their jurisdiction; designate their own heir in the case of hereditary titles; and mint standard Confederation currencies for use in trade outside the Confederation. Additionally, those of ducal rank have the ability to ennoble others up to the rank of Mandrinn, and each noble (at the Chancellor's discretion) may have a limited number of armed retainers, usually the equivalent of a non-'Mech regiment.
Magistracy of Canopus
- Frones 
- Duke 
- Star League Sourcebook, p. 23
- Classic BattleTech RPG, p. 171
- A Time of War, p. 246
- Star League Sourcebook, p.23
- House Davion (The Federated Suns), p. 99-102
- Handbook House Davion, p. 108-109
- House Steiner (The Lyran Commonwealth), p. 90
- Handbook: House Steiner, p. 96-97
- Handbook: House Kurita, p. 114 "Pillar of Gold (Government) - The Nobility"
- Handbook: House Kurita, p. 116 "Pillar of Gold (Government) - Privileges"
- Handbook: House Kurita, p. 117 "Pillar of Gold (Government) - Sociopolitical Structure - Noble Titles"
- Handbook: House Kurita, p. 116 "Pillar of Gold (Government) - Women and Titles"
- Classic BattleTech Companion, p.208
- Handbook: House Kurita, p. 104 "Pillar of Ivory (Culture and Philosophy) - The Art of Vendetta"
- House Marik: The Free Worlds League, p. 72-73
- Handbook House Marik, p. 88-89
- House Liao (The Capellan Confederation), p. 60
- Handbook House Liao, p. 94
- LosTech: The MechWarrior Equipment Guide, p. 24
- Threads of Ambition, Prologue
- The Periphery, 2nd Edition (sourcebook), p.32
- Handbook Major Periphery, p. 153