We review recent progress in potential theory of second-order elliptic operators and on the metastable behavior of Markov processes.There has been many recent progress in the potential theory of non-reversible Markov processes. We review in this article some of these advances. In Section 1, we present a brief historical overview of potential theory and we introduce the main notions which will appear throughout the article. In Section 2, we present two variational formulae for the capacity between two sets induced by second-order elliptic operators non necessarily selfadjoint. In the following three sections we present applications of these results. In Section 3, we discuss recurrence of Markov processes; in Section 4, we present a sharp estimate for the transition time between two wells in a dynamical system randomly perturbed; and in Section 5, we prove the metastable behavior of this process.