The phylogeny of the major groups of tetrapods (amphibians, reptiles, birds, and mammals) has until recently been poorly understood. Cladistic analyses of morphological data are producing new hypotheses concerning the relationships of the major groups, with a focus on the identification of monophyletic groups. Molecular phylogenies support some of these views and dispute others. Geological dates of the major evolutionary branching points are recalculated on the basis of the cladograms and new fossil finds.