This chapter focuses on the clinical findings and syndromes related to infarction at different locations within the vertebrobasilar circulation that are frequently related to large artery occlusive disease. Localization within the posterior circulation is simplified by dividing the vertebrobasilar territory into proximal, middle, and distal territories. Lesions within the medulla oblongata are most often unilateral and predominantly lateral tegmental. Lateral medullary infarcts are most often explained by intrinsic disease, atherosclerotic stenosis frequently with superimposed thrombosis, or arterial dissection of the distal extracranial vertebral arteries (ECVA) or the intracranial vertebral artery (ICVA). Bilateral distal ICVA occlusions rarely cause infarction limited to the middle intracranial posterior circulation territory. The ecology of the ischemia (risk factors) and definition of the location of the brain lesions help to identify the most likely vascular lesions and stroke mechanisms. Most patients with posterior circulation ischemia require non-invasive vascular evaluation.