Stochastic resonance, in which the signal and/or signal/noise ratio in a nonlinear system can be enhanced by the addition of random fluctuations (noise) of appropriate intensity, is discussed. By revealing the relationship of stochastic resonance to earlier research, and especially to work by Debye in the 1920s. the phenomenon is set in a broad physical context. It is shown that the traditional techniques of statistical physics, for example linear response theory, are applicable to stochastic resonance and their implications for its range of occurrence are discussed.