The Hedgehog signalling pathway is essential for numerous processes during embryonic development. Members of this family of secreted proteins control cell proliferation, differentiation and tissue patterning in a dose-dependent manner. Although the overall activity of the pathway is diminished after embryogenesis, recent reports show that the pathway remains active in some adult tissues, including adult stem cells in the brain and skin. There is also evidence that uncontrolled activation of the pathway results in specific types of cancer.