Three core ideas are at the heart of this book: relational expertise, the capacity to interpret problems with others; common knowledge, which consists of knowing what matters for professionals in other practices; and relational agency, which involves using that common knowledge to take action with others. These ideas are based in cultural-historical approaches to learning and change, and give coherence to the arguments presented. This is not a recipe book; the ideas are offered as resources for reflecting on and developing professional and research practices, and the conditions in which they occur.