Enlargement of the skull vault occurs by appositional growth at the fibrous joints between the bones, termed cranial sutures. Relatively little is known about the developmental biology of this process, but genetically determined disorders of premature cranial suture fusion (craniosynostosis) provide one route to the identification of some of the key molecules involved. Mutations of the MSX2, FGFR1, FGFR2, FGFR3 and TWIST genes yield new insights, both into normal and abnormal cranial suture biogenesis and into problems of broad interest, such as the conservation of molecular pathways in development, and mechanisms of mutation and dominance.