Effective use of geothermal energy requires the ability to move and convert heat efciently. In some instances, heat is used to do work, as in the generation of electricity. In other cases, heat is either concentrated or dissipated. Regardless of the application, an understanding of the behavior of uids and materials when heated or cooled, and the implications for energy balances, is the foundation for achieving an economically successful outcome for any geothermal application. This chapter provides an introduction to the elements of thermodynamics that are important for such considerations.