This chapter summarizes the current understanding of the evolution anddiversification of birds (around 10,000 species). New insights into this field have mainlycome from two fundamentally different but complementary sources of information: themany newly discovered Mesozoic bird fossils (paleontology) and the wealth of geneticanalyses of living birds at various taxonomic levels (phylogenetics). Birds have evolvedfrom theropod dinosaurs, with which they share some morphological characters. The earlyevolutionary history of the group is characterized by the extinctions of many major cladesby the end of the Cretaceous, and by several periods of rapid radiations and speciation.The Neornithes (modern birds) consists of three major clades: the Paleognathae (ratitesand tinamous), the Galloanserae (ducks, geese, swans, fowl-like birds), and Neoaves (theremaining 95% of all bird species). The relationships in Neoaves are difficult to resolve, buttwo clades are identified: Coronaves and Metaves. Uncertainties remain about the monophylyof Metaves, however. The Neornithes began to radiate in the Cretaceous and forimportant groups, such as passerines (around 50% of all bird species), the earliest diversificationstook place in the southern supercontinent Gondwana. The constraints primarilyposed by the adaptation to flight has led to a relatively uniform avian anatomy across taxa.Most morphological differences between bird groups are found in those body parts thatare directly involved in foraging and locomotion.