One of the most salient advances in vertebrate paleontology in recent decades has been the settling of the question of the origin of birds, a problem that has vexed evolutionary biologists since well before Darwin. To be sure, the consensus is not unanimous, and many details of this branch of the phylogenetic tree are yet to be worked out, but we now have a much clearer picture of this problem than we had a decade ago. Less settled, but equally stimulating, has been the controversy over the origin of flight in birds and other flying vertebrates. Was there a gliding stage? Did flight begin from the ground up or from the trees down? Were birds initially arboreal? What selective pressures drove the ancestors of birds to take advantage of the aerial opportunity?