This paper highlights the roles of property models in simulation and synthesis/design of products and processes. Property models play a service role in simulation and a service cum advice role in synthesis/design. The service role is played by property models when, for the selected property model, the corresponding model parameters are available. The advice role is played by property models in terms of definition of design targets and evaluation of design feasibility. In both cases, model selection and derivative information influences accuracy and efficiency of simulation/design results. The paper also highlights the need for alternative simulation-based synthesis/design approaches where aspects of modelling, simulation, synthesis and design are solved simultaneously. Two simultaneous approaches-one employing a generalised modular framework to define the combined problem as an optimisation problem, and the other employing reduction techniques and lumped variables for graphical (visual) design, are presented. Illustrative examples highlighting the various roles and synthesis/design approaches are also presented.