Neovascularization is an early and critical step in tumor development and progression. Tumor vessels are distinct from their normal counterparts morphologically as well as at a molecular level. Recent studies on factors involved in tumor vascular development have identified new therapeutic targets for inhibiting tumor neovascularization and thus tumor progression. However, the process of tumor blood vessel formation is complex, and each tumor exhibits unique features in its vasculature. An understanding of the relative contribution of various pathways in the development of tumor vasculature is critical for developing effective and selective therapeutic approaches. Several such agents are currently in clinical trials, and many others are under development. In this review, the mechanisms and factors involved in tumor blood vessel formation are discussed. In addition, selected novel classes of antivascular therapies, including those targeting tumor endothelial cells and other components of the tumor vasculature, are summarized.