The therapeutic utility of trastuzumab (‘Herceptin’) in breast cancer patients with tumours that overexpress erbB2 established the principle that targeted inhibition of specific signal transduction pathways can provide a new approach to cancer treatment. The ErbB family of protein tyrosine kinases, in particular the epidermal growth factor receptor (EGFR), are commonly overexpressed in many solid human tumours and EGFR was the initial target for a drug discovery programme seeking small molecule inhibitors of the EGFR tyrosine kinase (TK) enzyme activity. The description of the anilinoquinazoline class of potent and selective TK inhibitors led to several candidate drugs from this chemical class, for example gefitinib (‘Iressa’) and erlotinib (‘Tarceva’), which are being evaluated in breast cancer patients. Rapid advances in cancer molecular genetics have identified numerous potential drug targets associated with abnormal control of cell division either downstream of the ErbBs, for example Ras and MEK, or in erbB-associated signalling networks, like Src kinase, which affect the tumour cell motility and invasiveness. Candidate drugs for several of these targets are currently being evaluated; for example, the prenylation inhibitor AZD3409, a mimetic of the CAAX box of K-Ras, inhibits protein farnesyl and geranylgeranyl tranferases and a novel, selective, orally active Src kinase inhibitor AZD0530 have entered Phase I clinical trials and may have utility in breast cancer therapy.