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

The best known system is the French Ancien Régime (Old Regime), a three-estate system used until the French Revolution (1789–1799). The monarchy included the king and the queen, while the system was made up of clergy (the First Estate), nobles (Second Estate), peasants and bourgeoisie (Third Estate). In some regions, notably Scandinavia and Russia, burghers (the urban merchant class) and rural commoners were split into separate estates, creating a four-estate system with rural commoners ranking the lowest as the Fourth Estate. Furthermore, the non-landowning poor could be left outside the estates, leaving them without political rights. In England, a two-estate system evolved that combined nobility and clergy into one lordly estate with "commons" as the second estate. This system produced the two houses of parliament, the House of Commons and the House of Lords. In southern Germany, a three-estate system of nobility (princes and high clergy), knights, and burghers was used. In Scotland, the Three Estates were the Clergy (First Estate), Nobility (Second Estate), and Shire Commissioners, or "burghers" (Third Estate), representing the bourgeois, middle class, and lower class. The Estates made up a Scottish Parliament.

Today the terms three estates and estates of the realm may sometimes be re-interpreted to refer to the modern separation of powers in government into the legislature, administration, and the judiciary. Additionally the modern term of the fourth estate usually refers to forces outside the established power structure (evoking medieval three-estate systems), most commonly in reference to the independent press or media.