This book is meant to be a textbook for undergraduate, postgraduate and research students of Physics and Chemistry. It can also be used as a textbook for first year engineering students. The book includes Theories of Phase Transitions alongwith their Range of Validity. Topics such as Chemical Equilibrium and Saha lonization formula have also been included in the book. A chapter on Basic Concepts of Probability has been included which is of auxiliary nature and may be omitted by those who are acquainted with the Theory of Probability. An attempt has been made to emphasize the physical basis of the subject, but without undue neglect of its mathematical aspects. The book thus bridges the gap between highly mathematical works and the usual less rigorous formulations of the subject. Problems are given at the end of each chapter which are meant to be read as integral part of the text. These involve a number of applications and also serve to highlight techniques. Two important Appendices of Useful Functions and Volume of a Hypersphere have been added in third edition. Binomial Distribution Table has been included in Fourth Edition as Appendix F.