Leptomeningeal collaterals provide the primary source of perfusion to ischemic brain tissue following the onset of acute ischemic stroke and are becoming an important imaging biomarker for stroke therapy triage. Collateral circulation is predictive of infarct growth, end infarct volume, and response to endovascular therapy. The strength of the collateral circulation varies among patients and is partially dependent on genetic and modifiable risk factors. Collateral circulation may be assessed by standard angiographic techniques, including digital subtraction angiography, computed tomography and magnetic resonance (MR) angiography, as well as a growing array of advanced MR techniques including arterial spin labeling and dynamic MR angiography. Simple scoring systems are used to estimate the relative strength of the collateral circulation for a given patient, although there are some discrepancies in the predictive value of these systems. In this review, we discuss methods and techniques for determining the robustness of the collateral circulation and the role of the collateral circulation in acute ischemic stroke assessment and triage.