The advent of large-array superconducting biomagnetometer systems, including sophisticated operating and data processing electronics, has enabled practical patient studies evaluating the potential contribution of magnetoencephalography to clinical medicine. The superimposition of sources localized magnetically onto anatomic images, referred to as magnetic source imaging, has allowed physicians to conduct this evaluation in the content of their customary medical practice. This article summarizes the basic technology of magnetoencephalography and magnetic source imaging and includes brief descriptions of the various applications that are being studied with these new sensor systems, some of which are discussed in subsequent articles.