This chapter focusses mainly upon the classical mechanical evaluation of the partition function for a gas of structureless atoms, and begins with a discussion of energy equipartition. The role of interatomic interactions is examined using the grand partition function, as it enables a more convenient separation of the roles played by kinetic and potential energy terms. Expressions are developed for nonideal contributions to the thermodynamic pressure, entropy, and the internal, Helmholtz, and Gibbs energies of a gas in terms of virial coefficients and their first temperature derivatives. A discussion of distribution functions leads to the classical Liouville and Boltzmann equations, while a consideration of binary collision dynamics enables development of the form for the monatomic Boltzmann collision term. An introduction to nonequilibrium phenomena is provided by the derivation of the traditional transport equations for an atomic gas.