Brainstem infarcts comprise approximately 10% of all first ischemic brain strokes. The extrinsic vascular supply to the stem is complex. The intrinsic vascularization of the stem may be conceptualized in terms of four relatively constant and distinct vascular territories designated anteromedial, anterolateral, lateral, and dorsal (or dorsolateral). The anatomic structures found within each intrinsic territory determine the symptomatology associated with infarction of that territory. This territorial anatomy permits the knowledgeable physician to plan an MR imaging examination tailored to the patient's history and to predict the patient's neurologic deficits from the MR imaging findings.