The timing of the evolutionary radiation of the modern, or neornithine, birds is controversial. The fossil record has indicated that the radiation occurred mainly in the aftermath of the Cretaceous–Tertiary (KT) extinction event. However, recent estimates of lineage divergence times calculated from molecular data have instead indicated that most of the major clades of modern birds originated in the Cretaceous. Because the known Mesozoic fossil record of modern birds is poor, fossils from the early Tertiary provide the first opportunity to document the pattern of the radiation. The limited phylogenetic hypotheses produced to date and including this post-KT fossil material suggest that only some of the ‘more basal’ clades of modern birds were, most likely, present during the Mesozoic Era, and that the radiation of the ‘more derived’ landbird group of clades occurred in the early Tertiary. Copyright © 2001 John Wiley & Sons, Ltd.