We review stochastic resonance (SR), a counter-intuitive phenomenon in which the signal due to a weak periodic force in a nonlinear system can be enhanced by the addition of external noise. A theoretical approach based on linear response theory (LRn is described. It is pointed out that, although the LRT theory of SR is by definition restricted to the small-signal limit. it possesses substantial advantages in terms of simplicity, generality and predictive power. We outline the application of LRT to overdamped motion in a bistable potential, the most commonly studied form of SR. Two new forms of SR, predicted on the basis of LRT and subsequently observed in analogue electronic experiments, are described.