Semiclassical methods have been extensively used in a variety of physical problems, ranging from nuclear to cluster physics and from ballistic transport to interaction effects in nanostructures. We present a few physical examples in order to illustrate the way in which semiclassics can be adapted to deal with complex problems. Special emphasis is devoted to the problem of decoherence. When a one-particle system, whose classical correspondent is chaotic, evolves coupled to a weak quenched environment, we can show that the decoherence rate is asymptotically given by the mean Lyapunov exponent. Its independence of the perturbation strength, within a given range, is consistent with numerical simulations and recent experiments of spin echo in nuclear magnetic resonance.