Recently a large number of new anticancer agents targeting specifically one or more of the extracellular, transmembrane, or intracellular (but extranuclear) processes involved in malignant transformation of cells or carcinogenesis have been developed. These agents show target specificity, predominantly resulting in growth inhibition in tumor models and less frequently in tumor regression, acting in a cytostatic rather than a cytotoxic way. In addition, based on their specific mechanism of action, these target specific agents are expected to have a more favorable toxicity profile. In exploring new anticancer agents, phase I studies generally focus on toxicity and are primarily designed to describe dose limiting toxicity and to determine the maximum tolerated dose and the dose recommended for phase II studies. These phase II studies are subsequently performed in small groups of patients using the percentage tumor regression to screen for anticancer efficacy. Due to the anticipated low toxicity profile and the mainly growth inhibiting activity of target specific agents, the design of phase I and II studies involving these agents will have to be adapted in several ways. It is emphasized that, although it is helpful to distinguish cytotoxic from cytostatic anticancer agents, this dichotomy can be a simplification. In this paper, we will discuss important issues that will have to be faced when developing clinical trials with these agents and we will specifically translate this into the already known concepts of trial design exploring cytotoxic and cytostatic agents.