In the regulatory setting requiring evaluation of treatment effect, challenges arise due to multiple endpoints, multiple looks, missing data, change of study drug, and so on. Oncology clinical trials are unique and often open-label studies. Although overall survival is the gold standard, the primary efficacy endpoint in these studies varies depending on the disease stage of the patients being studied and the drug under consideration. The assessments of these endpoints, such as, tumor response rate and time to disease progression, commonly involve both subjective and objective measurements. Challenges in evaluating treatment effect under these conditions with select examples will be presented.