Stochastic ratchets provide a mechanism by which noise acting on a particle in a periodic potential can give rise to a non-zero particle current. They can be divided into two classes. In the correlation ratchet, the essential feature is that the noise is coloured. In the flashing ratchet, the potential switches between two forms and the noise may be white. Thus both types of ratchet are nonequilibrium systems, and there is no contradiction of the second law of thermodynamics. An additional requirement for a net flow of particles in one direction is that the potential be asymmetric. One reason why ratchets are important is that they give a possible explanation for protein motors. A few brave souls have even speculated that the principles involved might be used in the construction of nanoscale machinery.