Systematic Software Reuse (SSR) that is planned and repeatable is good for Software Engineering. The highest benefits are to be expected from reuse of large grain software artefacts, such as architectures. The book presents our architecture- centred Reuse Triplet concept, that equates SSR to plugging Reuse Software Components (RSC) into a Reuse Software Architecture (RSA). In general, the RSA and RSC will be reusable. We formally define and describe the three components of the Reuse Triplet, including some well-known software architectures. We present a formal description of plugging within the formal framework developed for describing RSAs. Finally, we look at a number of environments for architecture description as case studies of the Reuse Triplet concept. We critically examine some languages for their suitability for systematic software reuse under the Reuse Triplet concept. We contribute to the bag of tools that can be used in the effort to develop a strong culture of reuse that is based on a good understanding of what systematic reuse really entails at the technical level.