With the present emphasis on nano and bio technologies, molecular level descriptions and understandings offered by statistical mechanics are of increasing interest and importance. This text emphasizes how statistical thermodynamics is and can be used by chemical engineers and physical chemists. The text shows readers the path from molecular level approximations to the applied, macroscopic thermodynamic models engineers use, and introduces them to molecular-level computer simulation. Readers of this book will develop an appreciation for the beauty and utility of statistical mechanics.