A mere 5 to 7 years ago, the majority of literature on demyelinating, infectious, metabolic, and congenital diseases of the brain focused on comparison between computed tomography and magnetic resonance imaging (MRI). MRI has become not only the foremost diagnostic tool in imaging of the central nervous system, but also a key research instrument. This is displayed by the recent increase in papers concerning magnetic resonance spectroscopy. It is perhaps no better illustrated than in the study of multiple sclerosis. A review of neuroimaging in infectious diseases places a heavy emphasis on AIDS-related infections. The ongoing development of new scan sequences, contrast agents, and fast scanning techniques are broadening our image of the brain and, indeed, our understanding of pathophysiologic mechanisms of disease states. Excellent examples of this are the metabolic and congenital diseases where, based on the knowledge of metabolic pathways and embryology, MRI has become the modality of choice.