• Arteriovenous malformations (AVMs) of the brain are uncommon congenital lesions, causing symptoms in the middle decades of life. Neurosurgical and neuroradiological techniques have been developed to deal effectively with these lesions, in many instances resulting in a cure. This first part of a two-part article details the important anatomic aspects of these lesions, symptoms, and radiographic considerations. The largest group of lesions discussed here, the AVMs, vary in size but are usually large, involving important areas of the cortex and white matter with a complex array of arterial to venous shunts. Their most common initial symptoms are due to hemorrhage or seizure. The use of the computerized tomographic scan has often resulted in the unexpected diagnosis of these lesions. However, angiography remains the paramount method of defining these lesions.