Andrew McMahon is a leading embryologist who has provided critical insights into the role of cell signalling processes in the coordinated regulation of groups of cells during embryogenesis. In a series of impressive studies on the Wnt and Hedgehog proteins, he has uncovered common regulatory mechanisms involved in the development of the central nervous system and the digit patterning of vertebrate limbs. His work is notable for the powerful combination of biochemical, cellular and genetic techniques to explain how signalling centres active during embryogenesis coordinate the differentiation and patterning of developing tissues and organs.