In recent years the banking sector faces the problem that outdated Core Banking Systems (CBS), which have their technological roots in the 1970s and 1980s, hamper their technological and strategic development, especially in connection with ongoing trends such as cloud banking, omni-channel banking, and the "Digital Bank". Due to constantly added extensions and modifications CBS often became grown monolithic systems, with a large degree of complexity and an extensive technology stack, which render their modernization a difficult and resource-intensive task. To help overcome these problems, this book defines Core Banking Systems formally and explores them from a scientific perspective. Furthermore, it develops a generic, architectural modernization process, which shall help banks and CBS vendors to transform their systems into an updated state of the art framework, which satisfies their current and future strategic requirements. The process is built in a cyclic way and contains eight sub steps, which form an individual modernization lifecycle for each functional module of a CBS.