The cassock, also known as a soutane, is a long, sheath-like, close fitting, ankle length robe worn by Christian celebrants of various denominations, including Anglicans, Presbyterians and Roman Catholics. The cassock derives ultimately from the tunic that was formerly worn underneath the toga in classical antiquity.

The word cassock probably comes from the word "casaque" which means cloak; or cassaca, which means white.In older days it as known as vestis talaris.

It comes in a number of colours, which have traditional meanings. The ordinary priest's cassock is black. Bishops traditionally wear purple ones; cardinals, of course, get red ones, and the Pope's cassock is white. In tropical climates, ordinary priests frequently wear white cassocks.

The cassock comes in a number of styles, though no particular symbolism attaches to these. A Roman cassock has thirty-three buttons down the front; a French cassock has fewer front buttons, but buttons sewn to the sleeves after the manner of a suit, and a broader skirt. A Jesuit cassock has a fly fastened with hooks. An Anglican cassock is double breasted and fastens at the shoulders on the opposing side of the breast.

Cassocks are sometimes worn by lay people when they are assisting with the liturgy in church.They are also worn by altar servers, and by religious who are not priests, (e.g religious brothers,seminarians).In most Western countries, the clergy have generally abandoned the cassock as everyday clothing in favour of a clerical suit of more conventional design.