In order to develop new concepts into prototypes and ultimately into products, physical system modeling is virtually a necessity. At the concept stage, low order models are needed to understand the interactive dynamics of complex systems, and, as development proceeds into prototyping and manufacture, more sophisticated models may be needed to size components, determine fatigue life, plus more. As the product becomes more and more defined the modeling depends more and more on special purpose software packages that evaluate stress and strain, magnetic circuit design, fluid flow fields, etc. These packages require that the product is near final form, as the input files for these programs require details about the system that would not be known in the concept development stage. The modeling discussed in this paper is specifically directed to the concept development of mechatronic systems. These systems typically involve multiple energy domains where electro-mechanical,pneumatic, and -hydraulic devices are involved. Since the device or system is not well defined at this stage, the modeling must be handled by the inventors using physical principles, and assembling the various pieces of the model into an overall dynamic model that can be simulated. Bond graphs are particularly well suited for concept development of multi-energy domain systems. This is demonstrated here using several examples where modeling at an early stage of development would have avoided some very costly mistakes.