PDGF is one of the most potent serum mitogens, and the signalling mechanism by way of its receptor tyrosine-kinase has been extensively studied since its first purification in 1979. The identification of homology between the simian sarcoma virus oncogene, v-sis, and the B-chain of PDGF, as well as the frequent over-expression of both the ligands and receptors in various tumours and stroma led to the proposal of the PDGF-mediated autocrine and paracrine hypothesis. Consistent with the important roles of PDGF in the growth and survival of cells, the expression and activity of PDGF receptors are tightly controlled by both positive and negative feedback mechanisms at different levels. The deregulation of the control system can result in serious pathological conditions such as chronic inflammation and tumours. Understanding the molecular mechanisms for the regulatory system and the signalling pathway of PDGF is essential in order to find effective therapies in the diseases where PDGF is involved.