A number of assays are currently under evaluation for their potential usefulness in the selection of new chemotherapeutic agents or as predictive indicators for use in the design of optimal cancer treatment. Assays fall under the general categories of tumor cell survival, treatment-induced cellular damage, and determination of inherent tumor factors which includes tumor cell kinetics, tumor oxygenation, and measurement of specific biochemical systems. While technical advances to optimize these assays are continuing, possible inter- and intra-tumor cell variability to anticancer treatment modalities may complicate the interpretation and subsequent use of these assays. Regardless of their ultimate usefulness as clinical predictors, these assays will be extremely valuable in better characterizing and understanding the cell biology and biochemistry of human malignancies.