Patterns of bipolar or antitropical distributions occur in a diverse array of marine invertebrate, vertebrate, and plant groups in the eastern Pacific Ocean. Available geologic and paleontological evidence does not support vicariance as a process in the creation of these distributions but instead favors biotic interchange between hemispheres. Moreover, the timing of these events suggests several breaches (both northward and southward) of the tropics rather than a single event. The fossil record is extremely important in delimiting potential hypotheses and allowing correlation with vicariance events. The congruence of some interchanges with major regional tectonic activity and others with Pleistocene glaciations is not surprising and argues for a plurality of mechanisms. Extinction of endemic taxa following interchange among marine invertebrates is rare, and none of the antitropical distributions reviewed here suggests that the arrival of a taxon in the adjoining hemisphere resulted in the extinction of an endemic taxon. Instead, interchange and endemic taxa coexist. In contrast to the extinction patterns, the patterns of radiations are extremely diverse with some immigrant taxa undergoing remarkable radiations, whereas other taxa are represented by single species. Temperate nearshore rocky communities in both the northern and southern hemispheres appear to be mosaics of species that share common ancestry (because of interchange), are cosmopolitan, and have independent origins within the region. Although some communities appear to be organized around products of interchange (e.g., kelp forests of California and Chile), only the taxa have immigrated; linkages and interactions between species are independent and locally derived.