When looking back, we can say that the journey leading to this book started in the fall of 1994. At that time, the first author started to study the spare parts management at the Royal Netherlands Navy (RNN), which was part of a large maintenance project of the RNN and the University of Twente. Professor Henk Zijm suggested to use the METRIC type models of Craig Sherbrooke as described in his book of 1992. Shortly later, a Navy Officer, Jan Willem Rustenburg, started his PhD project on this topic, with Henk Zijm and the first author as supervisors. In this project, the METRIC type models were applied to cases at the RNN, an exact evaluation method for a general multi-echelon, multi-indenture system was developed, and problems with yearly budget constraints were studied. The cases showed the strength of the so-called system approach in comparison to a straightforward item approach. In 1999, the first author moved to Eindhoven University of Technology, and he started to collaborate with multiple Original Equipment Manufacturers (OEMs) that manage large spare parts networks to serve their customers. Over the years, many master and PhD projects were carried out with companies such as ASML, DAF, IBM, KLM, Marel, NedTrain, Oc´e, Philips Healthcare, and Vanderlande Industries. In the spare parts networks of most of these companies, it is not common to backorder demands when stockouts occur, as assumed in the METRIC type models. Instead, lateral transshipments and emergency shipments are applied in order to satisfy a demand as quickly as possible. This avoids long and costly downtimes of the manufacturing and transportation equipment that is supported by the spare parts networks. Further, it was important to deal with multiple customer classes, each with their own target system availability level. This motivated the start of the final project of the second author in 2002 for the postgraduate program Mathematics for Industry. In this project, we collaborated with ASML, an OEM of expensive lithography equipment for the semiconductor industry. Subsequently, this project was continued with a PhD project.