An introduction to metal-barrier-metal (MBM) electron tunneling devices is given. The introduction is oriented toward applications of tunneling devices as detectors of optical and infrared radiation. The theoretical back-ground of tunneling devices is briefly discussed, as is the historical development of MBM devices. Applications in which MBM devices are currently finding use are presented. The present problems with MBM devices are analyzed, and possible directions for further progress in perfecting MBM devices are explored. The process of fabrication of a particular type of MBM detector, the edge MBM device, is outlined. Measured performance of MBM detectors is covered briefly.