Colleagues are not only an integral part of many people’s lives; empirical research suggests that having a good relationship with one’s colleagues is the single most important factor for being happy at work. However, so far, no one has provided a comprehensive account of what it means to be a colleague. To address this lacuna, we have conducted both an empirical as well as theoretical investigation into the content and structure of the concept ‘colleague.’ Based on the empirical evidence that (...) we present in this paper, we argue that ‘colleague’ is a dual character concept that has both a descriptive and a normative basis for categorization. Its descriptive dimension is characterized by three features, according to which two people are colleagues if they work for the same institution and know each other, or if they work for the same institution and work in the same field. An independent normative dimension is revealed, which shows that, as colleagues, we are expected to fulfill substantial normative expectations. Understanding the expectations that are encoded in the very structure of this concept is crucial to lay the groundwork for an ethics of collegiality. (shrink)