This chapter aims to provide a synthesis of the diversification of damselfishes (Pomacentridae). After the highlights of two unique ecological specializations observed in damselfishes, i.e., the farming behaviour and the mutualistic relationship with giant sea anemones (clownfishes), this chapter discusses the uncertainty about the phylogenetic position of Pomacentridae within the ray-finned fishes. Then, recent advances in the systematic biology of the family are summarized. Among others, the delimitation of subfamilies and tribes are presented as well as some cases of polyphyletism and synonymization. Finally, the major periods of lineage diversification and the iterative nature of ecological diversification observed in damselfishes are delineated by summarizing recent works about fossil pomacentrids, time-calibrated phylogenies and eco-morphological data.