To properly understand MR angiography and its artifacts, one needs to be firmly grounded in the basic physics of MR flow. Concepts such as "flow-related enhancement" are crucial to a proper understanding of time-of-flight MR angiography. Phase contrast MR angiography draws from the same principles used to explain "even echo rephasing." This article not only provides a physical basis for MR angiography but also explains the flow phenomena observed on routine MR images.