Hedgehog proteins constitute one of a small number of families of secreted signals that have a central role in the development of metazoans. Genetic analyses in flies, fish and mice have uncovered the major components of the pathway that transduces Hedgehog signals, and recent genome sequence projects have provided clues about its evolutionary origins. In this Review we provide an updated overview of the mechanisms and functions of this signalling pathway, highlighting the conserved and divergent features of the pathway, as well as some of the common themes in its deployment that have emerged from recent studies.