The purpose of writing this book is to explain basic concepts of equilibrium statistical mechanics to the ?rst year graduate students in engineering departments. Why should an engineer care about statistical mechanics? Historically, statistical mechanics evolved out of the desire to explain thermodynamics from fundamental laws of physics governing behavior of atoms and molecules. If a microscopic interpretation of the laws of thermodynamics were the only outcome of this branch of science, statistical mechanics would not appeal to those of us who simply wish to use thermodynamics to perform practical calculations. After all, validity of thermodynamics has long been established. In thermodynamics, a concept of fundamental equations plays a prominent role. From one such equation many profound predictions follow in a completely general fashion. However, thermodynamics itself does not predict the explicit form of this function.Instead,thefundamentalequationmustbedeterminedempiricallyforeach system of our interest. Being a science built on a set of macroscopic observations, thermodynamicsdoesnotofferanysystematicwayofincorporatingmolecularlevel information, either. Thus, an approach based solely on thermodynamics is not suf?cient if we hope to achieve desired materials properties through manipulation of nanoscale features and/or molecular level architecture of materials.