Recent successes, such as the demonstration of a iK SRAM, have established epitaxial GaAs on Si substrates as a promising technology rather than a device designer's dream. For the first time we can seriously consider combining the individual electronic and optical properties of GaAs and Si within a single epitaxial structure. Applications for GaAs on Si range from those that simply utilize the Si as a low-cost, large-area passive substrate with superior strength and thermal conductivity to the long-sought multifunction integrated circuits where Si and III-V components are integrated within a single monolithic chip. This paper will attempt to provide a realistic appraisal of the potential applications of epitaxial GaAs on Si with emphasis on the special demands imposed by each application and barriers that must be circumvented.