The Peruvian Amazon has one of the highest concentrations of aquatic biodiversity in the world, with almost 1,000 fish species currently known, and more described every year. This book shows 502 images representing 274 fish species from the Alto Purús National Park, a remote wilderness area located in the Fitzcarrald region of southeastern Peru. Species richness - the number of distinct evolutionary lineages - is a fundamental measure of overall biodiversity. Species are basic structural and functional units of ecology and evolution. Accurate species identification is necessary to document genetic, physiological, and ecological aspects of biodiversity. Recognizing and naming species matters, no matter how subtle the differences may seem to the human eye. This book provides the most complete record of fish diversity in the Fitzcarrald region to date, and will be useful to ichthyologists, ecologists, biogogeographers, and aquatic resource managers working throughout greater Amazonia.