The present chapter deals with the characterization and description of phase transitions in metallic systems with characteristic size down to the nanometer range. In particular, the chapter focuses on the solid-to-liquid transition in nanometer-sized particles. After a short introduction to classical thermodynamics and to the way it copes with the general properties exhibited by nanometer-sized systems, a rapid overview of the state of the art in the field of the solid-to-liquid transition is given. The heterogeneous melting processes taking place in mesoscopic systems are discussed in terms of both classical thermodynamic and numerical simulation approaches. In the former case, attention is focused on the case of mesoscopic Sn particles, for which a relatively large amount of consistent experimental data exists as a consequence of previous calorimetric studies. In the latter case, the behavior of mesoscopic Cu particles is discussed.