Arteriovenous malformations (AVMs) are rare inborn errors of vasculogenesis and/or angiogenesis. They can be diagnosed early in life if the malformations are evident on the body surface. When AVMs are located in the brain or other deep tissues, they are diagnosed either incidentally after imaging initiated due to the occurrence of first symptoms/complaints or after sudden deterioration of AVMs, such as in cases of acute bleeding. AVMs can be managed by interventional radiology, surgical resection, and/or medication. Although AVMs are not tumors, their extremely aggressive growth and high blood flow, pose a massive health problem for the patients and a significant treatment challenge for the physicians. Localization and extent of AVMs are of utmost importance when evaluating their treatment options. While rough localization of the AVMs is usually clear, the exact estimation of their extent and operability is largely dependent on the experience of the physicians. We consider AVMs operable if their extent is limited to a specific area of tissue and complete surgical resection of the affected tissue would not result in loss of body function or the lost body function could be adequately replaced. If AVMs have a diffuse extent and their complete surgical removal would be associated with irreparable loss of function of the affected body area or would be incompatible with life, we classify these AVMs as inoperable. This chapter describes the basic facts about AVMs and provides details on classification, treatment, and outcomes.