Antibody conjugates have been the subject of active research for as long or longer than monoclonal antibodies have been available in practical quantities. The species conjugated have varied from fluorescent tags to radioisotopes, from phytotoxins such as ricin to enzymes capable of activating prodrugs, and from traditional chemotherapeutic agents to derivatives of potently cytotoxic natural products. All of these have shown some utility, whether as experimental tools or as clinically approved therapeutic or imaging agents. However, practical considerations for therapeutic applications have led to the pursuit of antibody-drug conjugates (ADCs) of potently cytotoxic natural products as the most active area of research. These ADCs will be the topic of this review, with an emphasis on the evolution of the most common ADCs currently in or approaching clinical trials.