The concept of treating solid tumors by inhibiting tumor angiogenesis was first articulated almost 30 years ago. For the next 10 years it attracted little scientific interest. This situation changed, relatively slowly, over the succeeding decade with the discovery of the first pro-angiogenic molecules such as basic fibroblast growth factor and vascular endothelial growth factor (VEGF), and the development of methods of successfully growing vascular endothelial cells in culture as well as in vivo assays of angiogenesis. However, the 1990s have witnessed a striking change in both attitude and interest in tumor angiogenesis and anti-angiogenic drug development, to the point where a remarkably diverse group of over 24 such drugs is currently undergoing evaluation in phase I, II or III clinical trials. In this review I will discuss the many reasons for this. These features, together with other recent discoveries have created intense interest in initiating and expanding anti-angiogenic drug discovery programs in both academia and industry, and the testing of such newly developed drugs, either alone, or in various combinations with conventional cytotoxic therapeutics. However, significant problems remain in the clinical application of angiogenesis inhibitors such as the need for surrogate markers to monitor the effects of such drugs when they do not cause tumor regressions, and the design of clinical trials. Also of concern is that the expected need to use anti-angiogenic drugs chronically will lead to delayed toxic side effects in humans, which do not appear in rodents, especially in short-term studies.