Function is the architectural construct through which a unit makes its capabilities accessible and operable within its environment. It defines the structured pathways of interaction that allow external agents—human or machine—to engage with the unit’s Value. Without Function, Purpose remains inert and Value cannot be realized, leaving the unit unable to participate in coordination or exchange. In Purpose Driven Design, Function is not an interface detail but a governed interaction surface—expressed through flows, protocols, and interaction patterns aligned with specific relational contexts such as exchange, collaboration, or compliance. This chapter establishes Function as a design construct that enables operability, constrains interaction, and supports composability, accountability, and systemic integration.