  Metastability is a physical phenomenon ubiquitous in first order phase
transitions. A fruitful mathematical way to approach this phenomenon is the
study of rare transitions Markov chains. For Metropolis chains associated with
Statistical Mechanics systems, this phenomenon has been described in an elegant
way in terms of the energy landscape associated to the Hamiltonian of the
system. In this paper, we provide a similar description in the general rare
transitions setup. Beside their theoretical content, we believe that our
results are a useful tool to approach metastability for non--Metropolis systems
such as Probabilistic Cellular Automata.
