The incorporation of molecular profiling into routine clinical practice has already been adopted in some tumor types, such as human epidermal growth factor receptor 2 (HER2) testing in breast cancer and KRAS genotyping in colorectal cancer, providing a guide to treatment selection that is not afforded by histopathologic diagnosis alone. It is inevitable that over time, with rapid advances in scientific knowledge, bioinformatics, and technology to identify oncogenic drivers, molecular profiling will complement histopathologic data to influence management decisions. Emerging technologies such as multiplexed somatic mutation genotyping and massive parallel genomic sequencing have become increasingly feasible at point-of-care locations to classify cancers into molecular subsets. Because these molecular subsets may differ substantially between each other in terms of sensitivity or resistance to systemic agents, there is consensus that clinical trials should be more stratified for or be performed only in such molecularly defined subsets. This approach, however, poses challenges for clinical trial designs because smaller numbers of patients would be eligible for such trials, while the number of novel anticancer drugs warranting further clinical exploration is rapidly increasing. This article provides an overview of the emerging methodologic challenges in the cancer genome era and offers some potential solutions for transforming clinical trial designs so they can identify new active anticancer regimens in molecularly defined subgroups as efficiently as possible.