Development of the functional specification is an activity in the build stage of the Forrester Product Marketing And Management (PMM) Model. The functional specification serves as a blueprint for the construction or update of a product or solution. Functional specifications align product management with product development, R&D, engineering or manufacturing before the build phase begins (see Inside The PMM Model: Build). In this report, we explain the role of a functional specification, and share best practices for its creation and use.