The mainstay of clinical antineoplastic chemotherapy is multiagent combinations, most of which were developed empirically. Because of the desire to speed research and decrease costs, there is increasing interest in moving new drugs into clinical trials in potentially active combinations based on preclinical testing data. Different mathematical models have been proposed for evaluating drug interactions, which can be classified as synergistic (combinations demonstrating greater than the additive activity expected from each agent alone), additive, or antagonistic (drugs showing less activity in combination than expected from the sum of each agent alone). Here, we briefly review some of the principles for testing cytotoxic drug interactions. We focus this review on application of the Combination Index method (as developed by Chou and colleagues) in the evaluation of drug interactions in cell culture assays.