The concept of concept plays a central role in philosophy, serving both as a subject of study in disciplines such as logic, epistemology, and philosophy of mind, and as a methodologically central notion for those who think that philosophy is essentially concerned with analysing, deconstructing, developing, or ameliorating concepts. But what exactly are concepts, and why have they become so significant in philosophy? The chapters of this volume explore critical moments in the history of the concept of concept, investigating why and how philosophers across different eras and cultures have addressed concepts' nature, acquisition, and relationship to the entities to which they apply. Spanning classical Greek to modern Western philosophies, and incorporating Chinese, Indian, and Islamic traditions, the volume examines concepts as means for categorizing the world – tracing their evolution from elements of thought to foundational components of reality, and the transformation of the concept into the key notion of philosophy.