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 economic benefit of modularization.