Simulation of 'reality' is always based on a model of that reality. The models that are used in the various simulation approaches can be ordered in a hierarchical fashion from atomic to macroscopic. In this book the hierarchy of models of physical reality are treated systematically and evaluated in relation to applications. The purpose is to give the 'simulant' insight into the possibilities and limitations of the models. This is the first book to authoritatively cover the broad scope of simulation applications from quantum mechanics to fluid dynamics. The emphasis is on the physics behind the molecular models in chemistry and biochemistry, with practical guidance for applications and sample programs provided. This book will appeal to advanced level undergraduate and graduate students studying physical and computational science, as well as those working in interdisciplinary fields with an interest in simulation.