Members of the transforming growth factor beta (TGF-β) family of multifunctional peptides are involved in almost every aspect of development. Model systems, ranging from genetically tractable invertebrates to genetically engineered mice, have been used to determine the mechanisms of TGF-β signaling in normal development and in pathological situations. Furthermore, mutations in genes for the ligands, receptors, extracellular modulators, and intracellular signaling molecules have been associated with several human disorders. The most common are those associated with the development and maintenance of the skeletal system and axial patterning. This review focuses on the mechanisms of TGF-β signaling with special emphasis on the molecules involved in human disorders of patterning and skeletal development. © 2003 Wiley-Liss, Inc.