Jacob Viner’s The Customs Union Issue, published in 1950, is the one undeniable classic in its field. The first part of this paper traces the development of Viner’s thinking on preferential trading arrangements, places his work in context, and clarifies his position on disputed issues. The second part considers the reception of his work, from the enthusiastic early reviewers to the international economists who further developed the theory of customs unions, to contemporary practitioners. While practitioners consistently misread Viner, these misreadings were scientifically fruitful, and there are reasons why fruitful science might flow from poor contextual history. Among contemporary international economists, the book has become a classic, marking off and justifying a field of enquiry.