System dynamics is still evolving.  This paper argues additional rigour is needed if system dynamics is to achieve its full potential in helping us understand complex behaviour of human activity systems.  It argues that a detailed appreciation of how systems engineers define, analyse, specify, manufacture, operate and support complex systems could inform the evolution of system dynamics even though there are significant differences between the two disciplines. The proffered approach integrates systems thinking, system dynamics modelling and systems engineering. This integrated approach enables group model building and building of exceedingly complex models through top-down design and careful management of the complexity introduced at each stage of the model-building process.  The approach promises to engender greater confidence that models developed using it work and are both necessary and sufficient representations of the real world.  The greatest potential gain accruing from application of this methodology is enhanced acceptance of system dynamics.