The mammalian target of rapamycin (mTOR), an atypical serine/threonine kinase, plays a central role in the regulation of cell proliferation, growth, differentiation, migration and survival. Dysregulation of mTOR signaling occurs in diverse human tumours, and can confer higher susceptibility to inhibitors of mTOR. Rapamycin and its derivatives, CCI-779 and RAD001 (designated rapamycins), specifically inhibit the function of mTOR, leading to inactivation of ribosomal S6K1 and inhibition of cap-dependent translation initiation through the 4E-BP1/eIF4E pathway. The overall effect is an accumulation of cells in the G1 phase of the cell-cycle, and potential apoptosis. Preclinical studies indicate that rapamycins are potent inhibitors of the proliferation of numerous tumour cell lines in culture and of murine syngeneic tumour models or human xenografts. RAD001 and CCI-779 are in phase I and II trials, respectively, as anti-cancer agents. These trials have demonstrated promising anti-cancer activity and relatively mild side effects of CCI-779. Emerging results suggest that inhibition of mTOR signaling can be exploited as a potential tumour-selective therapeutic strategy.