The aim of the book is to present the theoretical foundations of the Rasch model in an understandable way. Through a comprehensible concept, it aims to facilitate the entry for both students and practitioners. A key characteristic of this book is the use of open-source software. For practical implementation, the freely available program R is used, which allows for statistical calculations and the creation of graphics. A brief tutorial introduces readers to R, demonstrating the most important basic principles and guiding them from installation to data management. For the concrete implementation of analyses based on the Rasch model, the additional package eRm is used. Numerous examples provide detailed explanations of input and output.