  Several relevant aspects of quantum-field processes can be well described by
semiclassical methods. In particular, the knowledge of non-trivial classical
solutions of the field equations, and the thermal and quantum fluctuations
around them, provide non-perturbative information about the theory. In this
work, we discuss the calculation of the one-loop effective action from the
semiclasssical viewpoint. We intend to use this formalism to obtain an accurate
expression for the decay rate of non-static metastable states.
