The Fourth Edition of the Howard and Moore Checklist of the Birds of the World has been published in two volumes: Volume 1 - Non-passerines and Volume 2 - Passerines. This is a new edition of a major reference work and most respected scientific checklist of birds of the world. It has been updated to reflect the considerable change in our understanding of the evolution of birds derived from the study of their DNA over the past decade; thus the Cracraft sequence adopted in 2003 is completely revised here.The new edition also reflects revisions to species and subspecies. Evaluation, as before, is based on the Biological Species Concept, carefully and conservatively applied. Range statements have been revised, particularly for the Americas. Overall the number of footnotes and references cited about doubles.