This chapter outlines basic aspects of magnetoencephalography (MEG) technology, and provides an example of how the method is used clinically. In the clinical arena, MEG is most commonly used for presurgical planning purposes with the goal of localizing epileptogenic regions in patients with medically refractory seizures. In many of these cases, MEG provides unique clinical information that alters clinical care in a positive manner. Resective surgery was performed in the epileptogenic zone as originally identified by MEG and the patient has been seizure free for more than 2.5 years. The chapter describes how MEG is being used in sleep research. MEG is starting to play an increased role in the understanding of how brain circuits are modulated during sleep, with the most significant insights coming in relationship to the origins of sleep spindles and the complex modulation of neural interactions during rapid eye movement (REM) sleep.