A music-theoretic discussion of metric structure. Describing a musical passage as "metric" usually implies that one can hear in it an isochronous series of beats and that these beats are hierarchically structured. In some cases, however, one cannot infer a wholly isochronous metric structure from the durations present on the musical surface. In particular, there may be some meters where the beat level of the metric hierarchy consists of a nonisochronous series of durations; these cases are referred to as complex meters, A number of these complex metric structures are presented and discussed. The implications of these structures for various models of metric perception are then considered, with particular reference to their implications for the entrainment model proposed by Jones and Boltz (1989). It is proposed that such meters must be accounted for under an additive rather than multiplicative formalism. The paper concludes with some considerations of how entrainment to complex meters might be tested, as well as the ways in which experiments that focus on complex meters might provide insights into other aspects of temporal perception.