The escape from metastable states via noise-assisted hopping and/or tunnelling is pivotal to many scientific disciplines. It impacts on such diverse physical, chemical and biological processes as diffusion in solids, chemical reactions, nucleation phenomena and transfer of matter and information in biological systems. This volume surveys recent developments in the rate theory of both equilibrium and nonequilibrium processes. The understanding of the classical and quantum-mechanical concepts of this theory is deepened and extended in order to cope with various problems which, in particular, arise in complex systems. A wide range of applications are discussed such as correlated hops in periodic potentials, fluctuating barriers, transitions to limit cycles, discrete time dynamics, random walks on selfsimilar structures, and nonexponential decay in disordered systems is covered and profoundly discussed. This text is suitable for research workers and graduate students in chemistry, physics and biology with an interest in reaction rate theory.