In their search for regulators of animal growth and development, biologists have often come upon members of the transforming growth factor beta (TGF-beta) family and have realized that these are among the most versatile carriers of growth and differentiation signals. New evidence suggests that these factors signal through receptors with remarkable structures. Each receptor is a complex of two distantly related transmembrane serine/threonine kinases that are both essential for signalling. TGF-beta and related factors have at their disposal a repertoire of such receptors, a feature that could account for their multifunctional nature.