A reservoir simulator consists of a large set of models and parameters to describe the geology, fluid behavior, wells and surface facilities, and rules and conditions that describe and control the production process. On the numerical side, you have a combination of discretization methods, nonlinear solvers, linearizations, linear solvers, preconditioners, stability and convergence checks, and algorithms for automated time-step selection. The AD-OO framework in MRST is developed to encapsulate all these details, such that on one side it offers industry-grade simulator capabilities, and on the other side a flexible framework for rapid development of new proof-of-concept implementations. This chapter outlines the design philosophy behind AD-OO and explains many of its ingredients in detail. By reading this chapter, you will get an in-depth introduction to all the details you need to make a full-fledge reservoir simulator. The chapter ends with a discussion of three cases: a simple pressure depletion, multisegment representation of instrumented wells, and the full SPE 9 benchmark.