The idea of a photoelectric technique applied to differential astrometry has been around for about a decade. When developed such instruments promise increased accuracy and speed. Using linear detectors they can determine relative magnitudes and colors of all stars in the field at the same time that the relative positions are measured. Three such instruments have been proposed and have been developed to various degrees. These instruments and several variants will be reviewed. The current status of one instrument employing a rotating Ronchi linear grid, called the AMAS, will be discussed in detail. An accuracy of 5 μm in position is achieved routinely and proposed improvements project an improvement of a factor of ten to 0.5 μm. At this point the accuracy will be "seeing" limited and further improvement will be at the expense of observing time. The present instrument is sky limited at a magnitude of 11.5(V). A very minor change will increase this limit to 15.5(V) using two minute integration periods.