This paper will review some of the recent progress in GaAs-on-Si devices and circuits, and discuss the issues involved in realizing large scale ICs in GaAs-on-Si wafers. With a recent success in fabricating 1 Kbit static RAMs in GaAs-on-Si wafers, it has become apparent that this material is indeed acceptable for LSI complexity circuits. GaAs MESFETs fabricated using a standard process show an excellent threshold voltage uniformity which is comparable to that for bulk GaAs devices. GaAs bipolar devices on GaAs-on-Si have realized a gain of 25 which is the highest reported for bipolar devices in GaAs-on-Si material. Bipolar ring oscillators and 256-bit ROMs consisting of more than 100 gates have also been realized. In spite of these successes, however, there are numerous issues that need to be solved before this material becomes practical.