The Early Paleozoic (Cambrian-Devonian) witnessed a series of significant environmental changes including ocean-atmosphere oxygenation and progressive cooling due to a decline in CO2 levels. These changes were temporally associated with major radiations and extinctions of marine fauna and the establishment of complex ecosystem structure. Specifically, it is thought that an increase in shallow-water O-2 concentrations accompanied rapid diversification and colonization of marine invertebrates in the Cambrian and Ordovician, and expanded oceanic anoxia was linked with several extinction events. Temporal coincidence does not, however, imply causation, and many questions related to these relationships remain unanswered. Overall, a better understanding of the coevolution of animals and marine environments is expected through detailed exploration of their wideranging interactions during the Early Paleozoic. In this contribution, we review these topics in detail and thereby provide context for the contributions to this special issue.