  In this paper I will review some basic aspects of the mechanism of stochastic
resonance. Stochastic resonance was first introduced as a possible mechanism to
explain long term climatic variation. Since then, there have been many
applications of stochastic resonance in physical and biological systems. I will
show that in complex system, stochastic resonance can substantially change as a
function of the ``system complexity''. Also, I will briefly mention how to
apply stochastic resonance for the case of Brownian motors.
