The Hierarchical Approach is a systemic methodology for developing conceptual flowsheets for processes involving chemical reactions and separations. It can be easily applied independently of the use of a computing tool, but the best way is by association with the modelling environment of process simulation. The Hierarchical Approach consists of a top-down analysis organised as a clearly defined sequence of tasks aggregated in levels, each one handling a fundamental conceptual problem: (1) Chemistry and thermodynamics, (2) Input/Output structure, (3) Reactor-Separation-Recycles, (4) Separation system, (5) Energy and resources integration, (6) Environment, safety and hazard problems and (7) Plantwide control. At each level, systematic methods can be applied for the analysis and synthesis of subsystems. This chapter develops an improved methodology aiming to minimise the interactions between the synthesis and integration steps. The Input/Output analysis is extended to capture the essential of the ecological analysis. The structure called Reactor-Separation-Recycle dominates the conceptual frame of the whole flowsheet. Emphasis is on the material balance envelope formed by the subsystems of reactions and separations connected by recycles. This is the basis for setting-up the plantwide control strategy.The goal of this chapter is to present a systemic and systematic approach in developing flowsheets for chemical-like processes. The methodology is based on the Hierarchical Approach developed by J. Douglas in the late 1980s, and presented in his classical book about Conceptual Design of Chemical Processes (1988). The methodology, intended initially for petrochemical processes, attracted a much broader interest as a generic strategy for solving process synthesis problems for all technologies involving chemical reactions and separations. The conceptualisation of the methodology as a consistent and optimal process synthesis tool has been developed by Douglas and Stephanopoulos (1995). Variants of the 'Douglas doctrine' can be found in several textbooks about chemical process design, such as Dimian (2003), Smith (2005), Seider et al. (2009) and Turton et al. (2013).22Only the most recent editions are mentioned.The Hierarchical Approach is based on physical reasoning. Once understood, the methodology can be easily applied to a variety of situations independently of a computer simulation tool. However, the best way to take advantage of the innovative value is by association with the modelling environment of process simulation software.In the subsequent sections, we explain the approach in detail, illustrated by examples. To learn it properly, it is important that the reader goes through all the steps. Because of the complexity of the overall synthesis problem, some issues regarding the synthesis of subsystems, such as chemical reactors, separations or heat and energy networks, will be presented in separate chapters.