For more than a century, the concept of a "magic bullet" to deliver cytotoxic therapy to the site of disease has been envisioned but only recently have technological advances enabled antibody-drug conjugates to fulfill that dream. The recent approvals of brentuximab vedotin and ado-trastuzumab emtansine and emerging data for many molecules in clinical trials highlight the potential for antibody-drug conjugates to offer new therapeutic options for patients. This chapter reviews the evolution, state of the art, and potential future improvements that are enabling rapid development of this important class of cancer therapeutics.