In music, a catch is a type of round or canon at the unison. That is, it is a musical composition in which two or more voices (usually at least three) repeatedly sing the same melody, beginning at different times. Generally catches have a secular theme, though many collections included devotional rounds and canons.

In early collections the terms "catch" and "round" were interchangeable and, with part-songs and multi-voice canons, were all indexed as "songs". The catch and round differ from the canon in having a cadence on which the song can terminate after a specified number of repeats or when the leader gives a signal. A catch does not necessarily require the lines of lyrics to interact so that a word or phrase is produced from one part in the rests of another. This view became prevalent in the later part of the eighteenth century under the influence of the competitions sponsored by the Noblemen and Gentlemen's Catch Club.

Catches were originally written out at length as one continuous melody, and not in score. The change to printing in score was first made in the early eighteenth century, and this is now the normal method of presentation. In the score for a catch the different voices are usually labelled "1", "2", "3", etc. This indicates that voice "1" sings its part first and continues to part 2. When part 1 has been completed it is sung by voice "2" and so on. A common mistake in performance is for all parts to start together as though the score were to indicate a part song.