Course 3 of Statistical Thermodynamics, Ideal Gases, explores the behavior of systems when intermolecular forces are not important. This done by evaluating the appropriate partition functions for translational, rotational, vibrational and/or electronic motion. We start with pure ideal gases including monatomic, diatomic and polyatomic species. We then discuss both non-reacting and reacting ideal gas mixtures as both have many industrial applications. Computational methods for calculating equilibrium properties are introduced. We also discuss practical sources of ideal gas properties. Interestingly, in addition to normal low density gases, photons and electrons in metals can be described as though they are ideal gases and so we discuss them.