My research focuses on the evolutionary biology of Neotropical freshwater fishes. The rivers of  South and Central America harbor the most diverse freshwater fish fauna on Earth, with an estimate exceeding 6000 species or perhaps as many as 10% of all vertebrates. Despite this astonishing diversity, very little is known about its origin. I combine extensive fieldwork with molecular and morphological systematics, biogeography and various aspects of evolutionary biology to study the evolution of Neotropical fish diversity using the family Cichlidae as a model. Cichlid fishes are well-known models of adaptive evolution in lakes, but little is known about their diversification in rivers. Some of the questions I try to address include: Are there riverine cichlid adaptive radiations, or is this form of ecologically-driven, explosive diversification restricted to lakes? If riverine cichlids underwent adaptive radiations, when did they occur? What is the role of ecology in riverine cichlid diversification? What are the mechanisms of speciation in riverine cichlids? I believe Neotropical cichlids provide a particularly suitable model to study these questions. Furthermore, because cichlids are one of the three most-diverse families of Neotropical fishes, they also are ideal models for studying the evolution of Neotropical freshwater fishes. Comparative evolutionary studies of cichlids and other Neotropical fish groups should provide a powerful tool to understand the origin of the most diverse freshwater fish fauna on the planet.