This chapter provides background information on five different topics that are important for an understanding of the tentative decision support system presented in Chapter 2. Terms and definitions used in that text are introduced in this chapter. The first section introduces maintenance support services, focusing on concepts relevant for third party maintenance support provides within the defence intrustry. The next section discusses important concepts related to maintenance management, including the definition of the tactical decision level. The third section presents fundamental concepts of the maintenance optimization research area, which concerns the application of mathematical models to optimize maintenance performance. The fourth and fifth sections presents the basics of reliability modeling and simulation-based optimization, of which an understanding is important for the later presentation of the proposed reliability model and optimization method respectively.