Metastability is a phenomenon that occurs in the dynamics of a multi-stable non-linear system subject to noise. In models of statistical mechanics, it is related to the behavior of macroscopic quantities in the vicinity of a first order phase transition. A signature of metastable systems is the existence of multiple, well separated time scales. While at short time scales the system appears to be in a quasi-equilibrium (metastable state), at longer time scales rapid transitions between meta-stable states occur which are induced by random fluctuations. The understanding of the quantitative...