This book explains the ideas and techniques of statistical mechanics—the theory of condensed matter—in a simple and progressive way. The text begins with the laws of thermodynamics and the basic ideas of quantum mechanics. The conceptual ideas are then developed carefully, and the mathematical techniques are developed in parallel to give a coherent overall view. The text is illustrated with examples not just from solid state physics, but also from recent theories of radiation from black holes and recent data on the background radiation from the Cosmic Background Explorer. This second edition includes additional advanced material often found in undergraduate courses. It includes three new chapters on phase transitions at an appropriate level for an undergraduate student, and there are numerous exercises at the end of each chapter, along with brief model answers for the odd-numbered problems. It is a useful and practical textbook for undergraduates in physics and chemistry.