Publisher Summary This chapter presents a preliminary technology assessment of the impact of solid-state microwave devices. The problem of the regulation of microwave devices to ensure proper use of the electromagnetic spectrum is analyzed. The term microwave describes electromagnetic radiation spanning a particular portion of the spectrum of all radiation and the microwave band is generally taken to mean the frequency range 109 to something over 1011 Hz. The bandwidth and directionality properties of microwave radiation have determined the range of past applications. The efficiency of a transferred electron oscillator is dependent upon the ratio of the highest electron drift velocity achievable in the material to the lowest velocity, as the current is proportional to carrier velocity. It is found that a number of important microwave devices generate microwave energy by exploiting the time delays inherent in producing avalanche breakdown in a junction and the large numbers of mobile carriers produced in the avalanche.