Abstract: Magnetoencephalography (MEG) is a non- invasive functional brain imaging technique that detects and maps the magnetic fields generated by neural activities. It has a combination of ultra-high temporal resolution and relatively high spatial resolution. In this paper, the neurophysiological and biophysical basis of MEG signals is described. While sharing some similarities with electroencephalography, MEG has different distinct features. We first review the current traditional MEG system that employs a superconducting quantum interference device (SQUID) and related technologies. In response to its limitations, a new technique to detect magnetic fields in the brain has been developed based on the optically- pumped magnetometer(OPM). From this we observe that MEG is an irreplaceable functional brain imaging modality for both neuroscience research and clinical applications, and that OPM technology has the great potential to open up a new avenue for the future development of MEG.