Successful cancer treatments of the future are being developed with a focus on the molecular targets underlying the pathophysiology of neoplasia. Prominent targets which have emerged are those which are mutated in the course of a cancers development, and mediate activation or release from suppression of pathways mediating proliferation or apoptosis. These arguably are “pathogenic” targets. However, equally important are targets which can be defined on the basis of “large scale” analysis techniques of gene or protein expression in tumors which define targets expressed as a result of a tumors differentiation state or tissue of origin (“ontogenic” targets); targets mediating drug uptake or metabolism (“pharmacologic” targets), and “microenvironmental” targets mediating the alteration of tumor stromal elements. Irrespective of the nature of the molecular target which is the focus of new therapeutic efforts, target definition in susceptible tumors or patients ideally would be part of the development plan. In addition, an understanding of the therapeutic index which might be achieved in host vs tumor tissues using a surrogate or actual marker of drug effect ideally would be available from animal models and inform the development strategy in humans.