There is a need for obtaining low order approximations of high order models of physical systems as low order models result in several advantages including the reduction of computational complexity and improved understanding of the original system structure. In this book, some new approaches are provided for model reduction in the physical domain. The approaches that are presented use the idea of decomposition of physical systems which is useful for the identification of dominant components or subsystems. The procedures are applied to the physical systems that are represented by bond graphs as they lead to better understanding of the system structure. The provided decomposition and model reduction procedures are directly implemented on the model providing a better perception of the physical model reduction and a better design point of view. The advantages of the presented approaches over existing methodologies are emphasized through several examples. This book is especially for those who wants to obtain in depth information on bond graphs and physical systems and especially for practical engineers who wants to simplify their real systems for many purposes.