Function modelling is one of the most versatile and underused techniques in enterprise architecture — a hierarchical decomposition from mission to activities that brings clarity to scope, responsibility, and design. This white paper introduces function modelling and shows how it connects to goal modelling, process analysis, service design, and capability definition, providing a unified picture of how these paradigms relate and reinforce each other. It is a practical guide for architects, analysts, and anyone trying to make sense of what an organisation does and how to design what it should do next.