This classic text, first published in 1972, is designed for graduate physics courses in statistical mechanics. The second edition, published in 1996, incorporated three comprehensive chapters on phase transitions and critical phenomena. This third edition includes new sections on Bose-Einstein condensation and degenerate Fermi behavior of ultracold atomic gases, and two new chapters on computer simulation methods and the thermodynamics of the early universe. We have also added new sections on chemical and phase equilibrium, and expanded our discussions of correlations and scattering, quantized fields, finite-size effects and the fluctuation-dissipation theorem. We hope this new edition will continue to provide new generations of students with a solid training in the methods of statistical physics. -Bose-Einstein condensation in atomic gases -Thermodynamics of the early universe -Computer simulations: Monte Carlo and molecular dynamics -Correlation functions and scattering -Fluctuation-dissipation theorem and the dynamical structure factor -Chemical equilibrium -Exact solution of the two-dimensional Ising model for finite systems -Degenerate atomic Fermi gases -Exact solutions of one-dimensional fluid models -Interactions in ultracold Bose and Fermi gases -Brownian motion of anisotropic particles and harmonic oscillators