Imaging of the blood vessel lumen can be best achieved by injecting a dye into the vessel and then imaging the dye with Xray, CT, or MR technologies, but a lot of information can be gleaned by measuring flow through the vessels with ultrasound. Because of the increased risk of stroke with conventional angiography, CT angiography or MR angiography is often the first choice of investigation for imaging intracranial arteries, but doppler ultrasound is still frequently used to assess the carotid arteries. Interpreting the results from any modality requires a good knowledge of vessel anatomy. In this chapter we present a quick review of the anatomy of the circle of Willis and provide case examples of vessel stenosis and aneurysms using MRA, CTA, and doppler ultrasound.