Radiolabeled amino acids (AAs) are an important class of imaging agents for PET and SPECT that target the increased levels of AA transport by many tumor cells. System L AA transport has been a major focus of tracer development, and work in this field has led to several tracers that are effective for imaging brain tumors. Emerging data also support the use of certain radiolabeled AAs for neuroendocrine tumors and prostate cancer. Recently, new (18)F-labeled AAs have been developed that target transporters other than system L, including system A, glutamine, xCT, and cationic AA transporters. This review provides an overview of this class of molecular imaging agents and highlights the current status of oncologic imaging with radiolabeled AAs in terms of tracer considerations and key clinical applications.