Summary form only given. The maximum frequency at which a transistor can oscillate is a figure from which much of the high-frequency performance of the transistor can be predicted. For transistors of any given type, this measurement can be made easily in a circuit in which the tunedcircuit frequency and the feedback are the only two adjustable elements. The maximum frequency is in itself a useful transistor parameter. However, in many cases it is more desirable to determine the cutoff frequency of the transistor action, the so-called alpha-cutoff frequency, or fca. Since the maximum frequeng of osallation is that frequency at which the transistor has a unilateral-power gain of unity, the gain at lower frequencies can be directly related to the ratio of operating frequency to fmax. In addition, the efEaency of oscillation, which is the ratio of rf output power to total dc input power, may also be defined in terms of this frequency ratio. It is shown that the unilateral power gain varies at 6 db per octave. It is also found that for surface-barrier transistors the efficiency of oscillation is given approximately by the formula: Efficiency = -0.8 log10 (f/fmax).