Copper is an essential trace element that plays a critical role in a variety of basic biological functions, and serves as a key component in a number of copper-dependent enzymes that regulate such processes as cell proliferation, angiogenesis, and motility. A growing body of preclinical work has demonstrated that copper is essential to metastatic cancer progression, and may have a role in tumor growth, epithelial-mesenchymal transition, and the formation of the tumor microenvironment and pre-metastatic niche. As a result, copper depletion has emerged as a novel therapeutic strategy in the treatment of metastatic cancer. We present a review of the physiologic role of copper with a discussion of relevant enzymes of the copper proteome in both normal tissue and in cancer. We conducted a comprehensive review of the available preclinical data of several copper chelation agents, including penicillamine, trientine, disulfiram, clioquinol, and tetrathiomolybdate (TM), across a variety of tumor types. We also present the existing early phase clinical trial data for the use of the copper chelator TM in the treatment of breast cancer and other malignancies.