Detectable magnetic fields are associated with electrical activity in the brain that may be evoked by sensory stimuli. Neuromagnetic fields as weak as 20 femtotesla can be studied with modern SQUID detectors without recourse to magnetic shielding. Studies in our laboratory and elsewhere reveal that the field patterns may be sharply localized near the appropriate area of the cortex for visual, somatosensory, or auditory stimuli. The temporal features of the magnetic response to visual stimuli reveal aspects of the organization of the visual system of the brain.