Angiogenesis plays a key role in the growth and metastasis of malignant tumors. The main purpose of this chapter is to discuss the key factors associated with the activation of tumor angiogenesis and the mechanisms involved in the formation of tumor microvessels (MVs) and to consider the possibility of classifying tumor MVs based on their morphology and clinical significance. During malignant growth, hypoxia leads to the activation of hypoxia-inducible factor-1, which, together with other angiogenic stimuli, triggers a cascade of biochemical events, including the secretion of vascular endothelial growth factor, which affects the development of new vessels and the survival of immature blood vessels. The mechanisms of tumor MV formation include sprouting angiogenesis, vessel co-option, intussusceptive angiogenesis, vasculogenic mimicry, and vasculogenesis. Existing classifications of tumor MVs are based on their degree of maturity and do not reflect all their diversity. We propose a new classification system for tumor MVs, that is associated with their morphology and clinical significance. We showed that the most significant factors in terms of the risk of regional metastasis and disease relapse in both glandular and squamous cell carcinoma are atypical dilated capillaries, structures with partial endothelial linings, and DCs with weak expression of CD34, as well as capillaries in the solid tumor component in squamous cell carcinoma. A new classification of tumor MVs has been proposed, reflecting their relationship with clinical predictors of the risk of disease relapse. Further research in this direction is of not only theoretical but also practical interest.