Background— The collateral circulation plays a pivotal role in the pathophysiology of cerebral ischemia. Current knowledge of the collateral circulation remains sparse, largely because of prior limitations in methods for evaluation of these diminutive routes of cerebral blood flow. Summary of Review— Anatomic descriptions of the collateral circulation often focus on more proximal anastomoses at the circle of Willis, neglecting secondary collateral pathways provided by leptomeningeal vessels. Pathophysiological recruitment of collateral vessels likely depends on the temporal course of numerous compensatory hemodynamic, metabolic, and neural mechanisms. Subsequent endurance of these protective vascular pathways may determine the severity of ischemic injury. Characterization of the collateral circulation with advanced neuroimaging modalities that provide angiographic information and perfusion data may elucidate critical determinants of collateral blood flow. Such information on the status of the collateral circulation may be used to guide therapeutic interventions. Prognostication and risk stratification may also be improved by routine evaluation of collateral blood flow. Conclusions— Contemporary understanding of the collateral circulation may be greatly enhanced through further refinement of neuroimaging modalities that correlate angiographic findings with perfusion status, providing the basis for future therapeutic and prognostic applications.