The estates of the realm, or three estates, were the broad orders of social hierarchy used in Christendom from the Middle Ages to early modern Europe. Different systems for dividing society members into estates developed and evolved over time.

Today, the terms three estates and estates of the realm may sometimes be reinterpreted to refer to the modern separation of powers in government into the legislature, administration, and the judiciary.^{[citation needed]} The modern term the fourth estate invokes medieval three-estate systems, and usually refers to some particular force outside that medieval power structure, most commonly the independent press or the mass media.

During the Middle Ages, advancing to different social classes was uncommon and difficult, and when it did happen, it generally took the form of a gradual increase in status over several generations of a family rather than within a single lifetime.

One field in which commoners could appreciably advance within a single lifetime was the Church. The medieval Church was an institution where social mobility was most likely achieved up to a certain level (generally to that of vicar general or abbot/abbess for commoners). Typically, only nobility were appointed to the highest church positions (bishops, archbishops, heads of religious orders, etc.), although low nobility could aspire to the highest church positions. Since clergy could not marry, such mobility was theoretically limited to one generation. Nepotism was common in this period.

Johan Huizinga observed that "Medieval political speculation is imbued to the marrow with the idea of a structure of society based upon distinct orders". The virtually synonymous terms estate and order designated a great variety of social realities, not at all limited to a class, Huizinga concluded applying to every social function, every trade, every recognisable grouping.

There are, first of all, the estates of the realm, but there are also the trades, the state of matrimony and that of virginity, the state of sin. At court, there are the 'four estates of the body and mouth': bread-masters, cup-bearers, carvers, and cooks. In the Church, there are sacerdotal orders and monastic orders. Finally there are the different orders of chivalry.

This static view of society was predicated on inherited positions. Commoners were universally considered the lowest order. The higher estates' necessary dependency on the commoners' production, however, often further divided the otherwise equal common people into burghers (also known as bourgeoisie) of the realm's cities and towns, and the peasants and serfs of the realm's surrounding lands and villages. A person's estate and position within it were usually inherited from their father and his occupation, resembling a caste system. In many regions and realms, there also existed population groups born outside these specifically defined resident estates.

The estates of the realm were not merely descriptive or informal divisions. The legal and political status of a person was strongly dependent on his estate. Medieval law was usually not codified or uniform, but the sources of law were diverse. Different laws applied to different estates, and the estates held distinct privileges. A nobleman or cleric could not be sentenced by an ordinary court. One set of laws governed the conduct of the merchants, another governed the feudal system, and the church was governed by its own laws. The application of canon law, which originated in the church, was more widespread than in modern times. The expressions of these rights varied by country. For instance, in Sweden, the right to conduct international trade was subject to licensing and typically only given to burghers. In some countries, non-nobles were legally prohibited from wearing clothing that was considered too luxurious by sumptuary laws.