Using a Cre-mediated conditional deletion approach, we have dissected the function of Twist1 in the morphogenesis of the craniofacial skeleton. Loss of Twist1 in neural crest cells and their derivatives impairs skeletogenic differentiation and leads to the loss of bones of the snout, upper face and skull vault. While no anatomically recognizable maxilla is formed, a malformed mandible is present. Since Twist1 is expressed in the tissues of the maxillary eminence and the mandibular arch, this finding suggests that the requirement for Twist1 is not the same in all neural crest derivatives. The effect of the loss of Twist1 function is not restricted to neural crest-derived bones, since the predominantly mesoderm-derived parietal and interparietal bones are also affected, presumably as a consequence of lost interactions with neural crest-derived tissues. In contrast, the formation of other mesodermal skeletal derivatives such as the occipital bones and most of the chondrocranium are not affected by the loss of Twist1 in the neural crest cells.