Response to anticancer agents is often restricted to subsets of patients. The recognition of factors underlying this heterogeneity and the identification of biomarkers associated with response to drugs would greatly improve the efficacy of drug treatment. Platforms that can comprehensively map drug response in high-throughput ex vivo provide a unique tool to identify associated biomarkers and provide hypotheses for mechanisms underlying variable response. Such screens can be performed on cell lines and short-term cultures of primary cells to take advantage of the respective models' strength, which include, e.g., the ability to silence genes in cell lines and the "indefinite" supply of primary cells where clonal selection can be avoided. Cohorts of such samples represent the natural diversity of cancers, including rarer mutations and combinatorial patterns of mutations.We here summarize a simple and scalable method for the measurement of viability after drug exposure based on ATP measurements as a surrogate for viability, which we use to measure and understand drug response in cell lines and primary cells.