Developing new oncology drugs has increased since the improved understanding of cancer's complex biology. The oncology field has become the top therapeutic research area for new drugs. However, only a limited number of drugs entering clinical trials will be approved for use as standard of care for cancer patients. Molecular imaging is increasingly perceived as a tool to support go/no-go decisions early during drug development. It encompasses a wide range of techniques including radiolabeling a compound of interest followed by visualization with single photon emission computed tomography or positron emission tomography. Radiolabeling can be performed using a variety of radionuclides that are preferably matched to the compound based on size and half-life. Imaging can provide information on drug behavior in vivo, whole body drug target visualization, and heterogeneity in drug target expression. This review focuses on current applications of molecular imaging in the development of small molecules, antibodies, and anti-hormonal anticancer drugs.