Phase-shift keying (p.s.k.) is discussed as a modulation technique for transmitting digital data over radio circuits subject to fading. The modest bandwidth requirements of p.s.k. modulation suggest that it can not only alleviate spectrum crowding, but can also transmit traffic with fewer errors. The theoretical results presented here indicate, however, that the random phase perturbations inherent in fading radio signals cause unavoidable degradation in the performance of p.s.k. systems. Experimental data which partially support the theoretical results are cited, and comments relevant to the improvement of p.s.k. systems are included.