The assizes (/əˈsaɪzɪz/), or courts of assize, were periodic courts held around England and Wales until 1972, when together with the quarter sessions they were abolished by the Courts Act 1971 and replaced by a single permanent Crown Court. The assizes exercised both civil and criminal jurisdiction, though most of their work was on the criminal side. The assizes heard the most serious cases, most notably those subject to capital punishment or, later, life imprisonment. Other serious cases were dealt with by the quarter sessions (local county courts held four times per year), while the more minor offences were dealt with summarily by justices of the peace in petty sessions (also known as magistrates' courts).

The word assize refers to the sittings or sessions (Old French assises) of the judges, known as "justices of assize", who were judges who travelled across the seven circuits of England and Wales on commissions of "oyer and terminer", setting up court and summoning juries at the various assize towns.

The courts of assize were the English equivalent of the still-existing French Cours d'assise. The term is derived by Middle English assise < Old French assise ("session, legal action" – past participle of asseoir, "to seat") < Vulgar Latin *assedēre < Latin assidēre ("to sit beside, aside, elsewhere") < ad + sedēre ("to sit").

By the Assize of Clarendon of 1166 King Henry II established trial by jury by a grand assize of twelve knights in land disputes, and itinerant justices to set up county courts. Before Magna Carta was passed (enacted) in 1215, writs of assize had to be tried at Westminster or await trial at the septennial circuit of justices in eyre. The great charter provided for land disputes to be tried by annual assizes at more convenient places. This work soon expanded, becoming five commissions. In 1293, a statute was enacted which formally defined four assize circuits.

For centuries, many justices of the Court of King's Bench, those of the Court of Common Pleas, and barons of the Exchequer of Pleas in some seasons of the year travelled around the country contributing to five commissions: their civil commissions were those of assize and of nisi prius; their criminal law commissions were those of the peace, of oyer and terminer and for jail delivery (meaning clearing out of prisoners).

The second commission heard cases which plaintiffs sought to receive priority. From an Act passed in the reign of King Edward I plaintiffs (claimants) could file pleadings at Westminster for the court to issue a writ to summon a jury to Westminster to appoint a time and place for hearing the causes there, stating the county of origin. Such writs used the words and form of nisi prius (Latin: "unless before"). The writ called the parties to Westminster (on a longstop date) unless the king's justices had assembled a court in the county to deal with the case beforehand.

The commission of oyer and terminer, was a general commission to hear and decide cases. The commission of jail delivery required the justices to try all prisoners not yet tried by judges held in the jails.

Historically, all justices who visited Cornwall were also permanent members of the Prince's Council, which oversees the Duchy and advises the Duke. Before the creation of the Duchy, the Earls of Cornwall had control over the assizes. In the 13th century Richard, 1st Earl of Cornwall, feted as 'King of the Romans', moved the assizes to the new administrative palace complex in Lostwithiel but they later returned to Launceston.