The second principle accounts for the stability of systems, but its most natural statement in terms of entropy increases for isolated systems is too general to be practically useful. This leads to the definition of thermodynamic potentials, such as the Gibbs free energy, whose variations dictate the feasibility of transformations under given specific conditions. These potentials will form the main theme dealt with in this chapter. In preamble, the role of changes in chemical composition, due either to chemical reactions within the system or to exchange of matter with the surroundings, will be considered in order to arrive at the most general framework for the evolution criteria that will be presented. Finally, we will discuss the various ways the state of a system can be stable, paying particular attention to the notion of metastability.