The goal and system definition is the first phase of a life cycle assessment (LCA). It may seem trivial, but the LCA results are often strongly dependent on the choices made in this key phase. In the International Organization for Standardization (ISO) norm 14040 (described in Chapter 1), this phase is referred to as the goal and scope definition. In this book, we refer to it as the goal and system definition to highlight to the reader the importance of clearly delineating and describing a system. This phase consists of, firstly, describing the objectives-what is the purpose of the LCA, what are the results going to be used for, who is the audience, and who are the stakeholders? Secondly, the function of the system considered is analyzed to define a unit that represents this function. Different scenarios are described to achieve this functional unit (FU). Finally, the system boundaries are specified.