While PTHrP acts as a hormone when it is produced in excess by certain cancers, and perhaps also in lactating women, the foetus, and lower vertebrates, it seems clear that PTHrP is an important paracrine regulator of several tissue-specific functions. Its roles in smooth muscle relaxation, placental calcium transport, and bone development are becoming firmly established. However, it is likely to be an important player in the control of cellular growth and differentiation, although much of our understanding of this role to date comes from indirect evidence. The next decade will be an exciting time in defining further both the hormonal and paracrine actions of PTHrP.