This book focuses on the development of multi-variant products using modular product structures and thus addresses the reduction of complexity from a product development perspective. These modular product structures allow for a greater variety of demand with a smaller, internal variety of components and processes. As a supplement to the common product development methodology, the necessary basics of modularity and variant diversity as well as the corresponding methods are presented comprehensively. The book thus summarizes the current state of science as well as the research activities of the past ten years at the Institute of Product Development and Design Technology at the TU Hamburg-Harburg. The target groups This book is aimed at product developers and decision makers in practice. Science is offered a helpful reference book and interested engineering students can immerse themselves in the development of modular product families with the necessary basics. This book is a translation of the original German 1 edition by Dieter Krause & Nicolas Gebhardt, published by Springer Fachmedien Wiesbaden GmbH, part of Springer Nature in 2018. The translation was done with the help of artificial intelligence (machine translation by the service DeepL.com). A subsequent human revision was done primarily in terms of content, so that the book will read stylistically differently from a conventional translation. Springer Nature works continuously to further the development of tools for the production of books and on the related technologies to support the authors.