The approval in 2004 of bevacizumab (Avastin), a neutralizing monoclonal antibody directed against vascular endothelial growth factor (VEGF) as the first anti-angiogenic systemic drug to treat cancer patients validated the notion introduced 33 years earlier by Dr. Judah Folkman, that inhibition of tumor angiogenesis might be a valid approach to control tumor growth. Anti-angiogenic therapy was greeted in the clinic a major step forward in cancer treatment. At the same time this success recently boosted the field to the quest for new anti-angiogenic targets and drugs. In spite of this success, however, some old questions in the field have remained unanswered and new ones have emerged. They include the identification for surrogate markers of angiogenesis and anti-angiogenesis, the understanding about how anti-angiogenic therapy and chemotherapy synergize, the characterization of the biological consequences of sustained suppression of angiogenesis on tumor biology and normal tissue homeostasis, and the mechanisms of tumor escape from anti-angiogenesis. In this review we summarize some of these outstanding questions, and highlight future challenges in clinical, translational and experimental research in anti-angiogenic therapy that need to be addressed in order to improve current treatments and to design new drugs.