**1. Introduction**

Endovascular strategies for addressing intracranial and extracranial diseases continue to gain momentum. These techniques are limited principally by technology and imagination. As newer devices and implements are introduced to the endovascular surgeon, more diseases previously construed to be the realm of open surgery or untreatable are becoming amenable to endovascular interventions. Because of the nature of endovascular procedures, with liquid agents, flow-directed therapies, and embolic materials, it is critical for a neuro-interventionalist to be aware of the collaterals that exist between the vessels being embolized and other critical collaterally connected vessels, occlusion of which may result in undesirable outcomes. Similarly, for other occasions, such collaterals may provide unique conduits that may afford access in novel ways to the intracranial or extracranial circulation. The understanding of these collaterals is best undertaken with an initial understanding of the development of the cranial vasculature. The rich anastomotic connections and interlinked development shed great light and provide a firm basis for understanding the cranial collaterals. Secondly, the collateral circulation may be divided by collaterals between extracranial and intracranial systems on one hand and collaterals between the internal carotid and vertebrobasilar (VB) systems on the other. In this chapter, we endeavor to provide a brief overview of cranial vascular development, followed by specific clinically relevant examples of extracranial and intracranial anastomoses and the internal carotid artery (ICA) and vertebrobasilar (VB) anastomoses, intracranial and intracranial anastomoses.
