Effective and efficient product development is critical to business success on the increasingly competitive global market, and simulation has proven to support this in many sectors. The aim of this thesis is to study how properties of complex mechanical and mechatronic systems can be more efficiently and systematically predicted, described, assessed and improved in product development. The purpose is to elaborate an approach that can, rather than only verifying solutions that are already decided upon, support dialogues with customers, stimulate creation of new concepts and provide guidance towards more optimised designs, especially in early development stages. This is here termed simulation-driven design. To be useful for this, product models and simulation and optimisation procedures must be efficient, that is, they must accurately answer posed questions and point towards better solutions while consuming an acceptable amount of time and other resources. In this thesis a coordinated approach to create such efficient decision support is elaborated. This is done by action research through two industrial case studies; an automobile exhaust system representing a complex mechanical system and a water jet cutting machine representing a mechatronic system. The general knowledge gained from these case studies should be a good base for coming implementation of this approach as an inherent working routine in companies developing complex mechanical and mechatronic products. A specific result is a validated virtual model of the exhaust system, which facilitates fast structural dynamics simulation of customer proposed design layouts. It is also shown that the non-linear flexible joint between the manifold and the rest of the exhaust system makes the system behaviour complex. This has resulted in an additional general research question, namely how systems that are linear, except for small but significant non-linear parts, can be simulated in an efficient way. Another specific result is a validated real-time virtual machine concept for simulation of the water jet cutting machine, which facilitates early-stage design optimisation. As the mechanics and the control system are considered simultaneously, interaction effects can be utilised. An introductory optimisation study shows a significant potential for improved manufacturing accuracy and a more light-weight design. This potential would not likely have been found through a conventional sequential design approach. The results of this thesis indicate that there is a great potential for improved product development performance in small and medium-sized companies. By incorporating modern simulation support these companies can improve their competitiveness as well as contribute to improved resource efficiency of society at large. In doing so, it is important to find a good balance between model fidelity, validity and cost for achieving a relevant decision support. The coordinated approach to simulation-driven design elaborated in this thesis is a promising and systematic way of finding this balance.