<p>Conceptual engineering holds that our concepts can be evaluated and improved. It faces many challenges, and a functional approach is promising at solving these challenges. However, it remains unclear and controversial what precisely qualifies as a concept’s function. In this paper, I develop an interest-centered theory of function, arguing that our understanding of function is interest-relative and system-based: the function of an item is the contribution it makes to a system capacity that is the object of legitimate interest. The theory is more worthy of our acceptance because it can accommodate many other function theories. Applying it to concepts clarifies several important issues in conceptual engineering. It offers a unified and theoretically grounded framework that addresses core challenges for functional approaches to conceptual engineering.</p>