Functional brain imaging using MRI (functional MRI or fMRI) has become a valuable tool for studying function/structure relationships in the human brain in both normal and clinical populations. This paper describes the physiological changes associated with brain activity, including changes in blood flow, volume, and oxygenation. The latter of these, known as blood oxygenation level dependent (BOLD) contrast, is the most common approach for functional MRI, but it is related to brain activity via a variety of complex mechanisms. Here we describe a physiological and fluid mechanical model that can explain the shape and behavior seen in the BOLD response and may allow for improved quantification of the measured response. We also describe basic acquisition methods used in BOLD fMRI and associated artifacts. Finally, we briefly introduce approaches to acquired alternate contrast mechanisms in fMRI.