A term expresses a thick concept if it expresses a specific evaluative concept that is also substantially descriptive. It is a matter of debate how this rough account should be unpacked, but examples can help to convey the basic idea. Thick concepts are often illustrated with virtue concepts like courageous and generous, action concepts like murder and betray, epistemic concepts like dogmatic and wise, and aesthetic concepts like gaudy and brilliant. These concepts seem to be evaluative, unlike purely descriptive concepts (...) such as red and water. But they also seem different from general evaluative concepts. In particular, thick concepts are typically contrasted with thin concepts like good, wrong, permissible, and ought, which are general evaluative concepts that do not seem substantially descriptive. When Jane says that Max is good, she appears to be evaluating him without providing much description, if any. Thick concepts, on the other hand, are evaluative and substantially descriptive at the same time. For instance, when Max says that Jane is courageous, he seems to be doing two things: evaluating her positively and describing her as willing to face risk. Because of their descriptiveness, thick concepts are especially good candidates for evaluative concepts that pick out properties in the world. Thus they provide an avenue for thinking about ethical claims as being about the world in the same way as descriptive claims. -/- Thick concepts became a focal point in ethics during the second half of the twentieth century. At that time, discussions of thick concepts began to emerge in response to certain disagreements about thin concepts. For example, in twentieth-century ethics, consequentialists and deontologists hotly debated various accounts of good and right. It was also claimed by non-cognitivists and error-theorists that these thin concepts do not correspond to any properties in the world. Dissatisfaction with these viewpoints prompted many ethicists to consider the implications of thick concepts. The notion of a thick concept was thought to provide insight into meta-ethical questions such as whether there is a fact-value distinction, whether there are ethical truths, and, if there are such truths, whether these truths are objective. Some ethicists also theorized about the role that thick concepts can play in normative ethics, such as in virtue theory. By the beginning of the twenty-first century, the interest in thick concepts had spread to other philosophical disciplines such as epistemology, aesthetics, metaphysics, moral psychology, and the philosophy of law. -/- Nevertheless, the emerging interest in thick concepts has sparked debates over many questions: How exactly are thick concepts evaluative? How do they combine evaluation and description? How are thick concepts related to thin concepts? And do thick concepts have the sort of significance commonly attributed to them? This article surveys various attempts at answering these questions. (shrink)