This is an introductory book which explains the foundation of the subject and its application. It is intended primarily for undergraduate students but may provide useful information and reading to science & engineering students but may provide useful information and reading to science and engineering students at all levels. It is assumed that readers have knowledge of basic thermodynamics and quantam mechanics. There are two approach in statistical mechanics: one based on the study of independant particles in an isolated system and other based on the concept of ensembles. In this book, attempt has been made to take advantage of both approaches. While fundamental concepts have been developed by first appraoch , concept of esembles have been included to bring out the importance of thuis concept in the application of statistical thermodynamics to chemical system where inter particle interactions become important.