Ergativity refers to patterning in a language whereby the subject of a transitive clause behaves differently to the subject of an intransitive clause, which behaves like the object of a transitive clause. Ergativity can be manifested in morphology, lexicon, syntax, and discourse organisation. This article overviews what is known about ergativity in the world's languages, with a particular focus on one type of morphological ergativity, namely in case‐marking. While languages are rarely entirely consistent in ergative case‐marking, and the inconsistencies vary considerably across languages, they are nevertheless not random. Thus splits in case‐marking, in which ergative patterning is restricted to certain domains, follow (with few exceptions) universal tendencies. So also are there striking cross‐linguistic commonalities among systems in which ergative case‐marking is optional, although systematic investigation of this domain is quite recent. Recent work on the diachrony of ergative systems and case‐markers is overviewed, and issues for further research are identified.