This paper proposes a definition for microphone efficiency, together with a discussion for justifying the points of view presented. Up to the present time, the question of microphone efficiency has been generally ignored and in its place has grown the use of microphone sensitivity, which is the voltage output for a particular value of sound pressure actuating the unit. Although this latter quantity may be quite satisfactory for commercial use, the author feels that from a scientific point of view, we should be interested in the absolute ability of a microphone to absorb acoustic energy from a sound field and convert it into electrical energy: this ability he defines as efficiency. A relation for microphone efficiency is derived and a family of curves are computed which show the efficiency of a microphone as a function of its size, impedance, and sensitivity. One obvious conclusion that can be deduced from the paper is that only a tiny fraction of the acoustic energy intercepted by an average microphone is actually converted into electrical energy. This means that we are yet very far from the theoretical limit and, consequently, great possibilities still exist for fundamental research in the improvement of microphone efficiency.