The transforming growth factor ß (TGF-ß) family of hormonally active polypeptides have attracted much attention because of their ability to control cellular functions that underwrite animal embryo development and tissue homeostasis. TGF-ß family members act by modifying the expression of specific sets of target genes, and biologists pursuing the elucidation of TGF-ß signaling mechanisms have turned up a fairly simple system, linking membrane TGF-ß receptors to such genes (for recent reviews, see Heldin et al. 1997; Massagué 1998; Whitman 1998; Massagué and Wotton 2000). If a TGF-ß signaling system can be so simple, and yet so powerful, then an elaborate network of regulators must keep control over the inputs, activity, and outcomes of this system. A multitude of regulatory mechanisms have been recently uncovered that control the access of TGF-ß family members to their receptors, the activity of their receptors and receptor substrates, and the nuclear function of the transcriptional complexes generated by this pathway. The regulatory mechanisms operating in the prereceptor phase of a TGF-ß signaling pathway can be as intricate and physiologically important as those operating downstream of TGF-ß receptors. These control mechanisms, which are central to understanding the physiology of TGF-ß signaling, are reviewed here.