Collateral circulation is a fundamental determinant of stroke pathophysiology. Distal arterial embolism and hypoperfusion resulting from severe proximal arterial stenosis may be offset by collateral flow. Collaterals influence whether or not infarction results. The detection and characterization of arterial deoxygenation and other consequences of collateral perfusion depend on neuroimaging techniques. Imaging advances will further the understanding of clinical correlates, including collateral sustenance and collateral failure, and possibly promote the development of collateral therapeutics. Refinements of perfusion imaging protocols may quantify the delay and dispersion of collateral flow more accurately. This review explores the role of collateral flow in acute ischemic stroke and describes the imaging modalities used to investigate phenomena "beyond the clot."