A text for upper undergraduate or first-year graduate courses in physics. Also useful in the personal library of scientists and engineers. This text treats problems with varying degrees of sophistication, and with several complementary approaches, illustrating principles with specific problems rather than rigorous formalism. Problems are solved from the points of view of kinetic theory, statistical mechanics of closed systems, statistical mechanics of open systems, and thermodynamics. These approaches sometimes lead to the same results, and sometimes to results which vary in generality, rigor, and predictive power.