Systemic cytotoxic (anti-proliferative) anticancer drugs rely primarily for their therapeutic effect on cytokinetic differences between cancer and normal cells. One approach aimed at improving the selectivity of tumour cell killing by such compounds is the use of less toxic prodrug forms that can be selectively activated in tumour tissue (tumour-activated prodrugs; TAP). There are several mechanisms potentially exploitable for selective activation. Some utilise unique aspects of tumour physiology such as selective enzyme expression, hypoxia, and low extracellular pH. Others are based on tumour-specific delivery techniques, including activation of prodrugs by exogenous enzymes delivered to tumour cells via monoclonal antibodies (ADEPT), or generated in tumour cells from DNA constructs containing the corresponding gene (GDEPT). Because only a small proportion of the tumour cells may be competent to activate the prodrug, whichever activating mechanism is used, TAP need to be capable of killing activation-incompetent cells as well via a "bystander effect", in order to fully exploit these "activator" cells. A wide variety of chemistries have been explored for the selective activation of TAP. These include reduction of quinones, N-oxides, nitroaromatics and metal complexes by endogenous enzymes or radiation, amide cleavage by endogenous peptidases, and metabolism by a variety of exogenous enzymes, including phosphatases, kinases, amidases and glycosidases.