The development of modular product families holds enormous economic potential for companies, as there are always great opportunities but also risks associated with all life phases of a product. However, these fundamental and far-reaching effects inevitably lead to conflicting objectives when defining modular product structures, which makes decision-making in product development particularly complex. Considering relevant theories from decision theory and product family design, this book presents an innovative method to support decision-makers in the development of modular product families. The central element of the method is a novel Modularity Decision Dashboard (MDD), which interactively visualizes all decision-relevant data. The findings presented here confirm that applying the method to real-world decision-making problems leads to a more balanced ratio between internal and external variety, and thus significantly contributes to the efficient use of economic benefit of modularization. The Author Marc Windheim studied Mechanical Engineering and Management at the Technical University of Munich and Stanford University. During his studies he chiefly focused on Systematic Product Development and Business Management. From 2014 to 2019, he worked for Hilti as a project manager, where he oversaw technology and platform projects for electric drives. At the same time, he was writing his dissertation on decision-making in the development of modular product families at the Institute of Product Development and Mechanical Engineering Design (PKT), Hamburg University of Technology. Since 2019, Marc has been working for Siemens Management Consulting as a Senior Consultant in the areas of Research & Development and Product Lifecycle Management.