The development of superior treatment strategies directed against metastatic breast cancer remains a major challenge. Because this condition is historically treatable but not curable, the primary goal of clinical management is often to create an optimal equilibrium between maximizing response, minimizing toxicity, improving survival, and optimizing symptom palliation. The search for new drugs that might provide these characteristics, together with an expanding understanding of the molecular basis of cancer, has prompted the development and investigation of “biologic” agents, which offer high tumor selectivity together with a favorable therapeutic ratio.