The interactions between avian obligate interspecific brood parasites and their hosts provide tractable systems for studying coevolutionary processes in nature. This review highlights recent advances in understanding coevolution in these systems. First, we discuss the evolution and phylogenetic history of avian brood parasitism. Next, we examine coevolved adaptations and counteradaptations in brood parasites and hosts at all stages of the host nesting cycle: those that precede laying of the parasitic egg and those at the egg, chick, and fledgling stages. We then consider the factors that affect the evolution of offense and defense portfolios (the suites of adaptations and counteradaptations across the nesting cycle), and the outcomes of coevolutionary interactions between brood parasites and hosts. Ongoing efforts to document the diversity of host defenses and parasite offenses will facilitate understanding of coevolutionary processes and the ecological and evolutionary consequences of species interactions in the natural world.