Functional brain imaging refers to a class of techniques that noninvasively measure correlates of neural activity. Positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) are the two technologies most commonly used today to study the human brain "in action." The explosion of information about human brain function occurring in the past decade has resulted in large part from these two techniques. As will be described in this chapter, PET imaging has made considerable contributions to our understanding of the mechanisms of drug action, mostly through application of radiopharmaceutical labeling of neurotransmitter receptors. fMRI, on the other hand, has gained rapid acceptance because of the widespread availability of magnetic resonance imaging (MRI) scanners, the lack of radioactive exposure, and the better image resolution offered.