The authors argue that an effective way to describe the manifestation of interest patterns within a particular work domain is through a nuanced description of interests in terms of the essential functional activities common to that domain. Focusing on the domain of business work and studying a large sample of business professionals over a 15-year period, the authors derived an eight-factor business core function model representing the way interest patterns are manifested in actual business work. This model is the basis for a business interest assessment instrument, the Business Career Interest Inventory (BCII), which has been used by more than 75,000 business professionals and business students and has become the career interest assessment tool of choice at more than 200 of the leading business schools around the world. This article describes the rationale for the core function model and presents basic psychometric information on the BCII.