Agreement is defined as the systematic covariance of one element with another. The most uncontroversial agreement configuration is that between a controller—an element intrinsically specified for a value of an agreement feature—and the target of agreement—the element reflecting a displaced feature value of the controller. The distribution of morphological agreement markers is however much wider than controller–target configurations: targets can express agreement values for features that are not visible on the controller and even show agreement morphology in the absence of a lexical controller. A second source of variation is due to the fact that in certain contexts there is a choice between syntactic agreement (with formal features of the controller) and semantic agreement (with semantic features of the referent of the controller). The choice between syntactic and semantic agreement is correlated in part with cross-linguistically observed regularities that have been formulated as the agreement hierarchy and the animacy hierarchy. Agreement morphology harnesses the same morphological devices found with derivation and inflection. Like inflectional morphology more generally, agreement morphology is only present in a subset of the world’s languages.