Book excerpt: The classification of products is a key element in international trade. The Harmonized System (HS) of the World Customs Organization (WCO) provides an internationally recognised system of product classification. It is used for many purposes, including import duties, rules of origin, freight documents and statistics. Fish is often one of the most complex commodities to classify, with several specificities. There is a multiplicity of phyla, species, and treatments, which create an intricate set of different layers covering different products and and make it especially difficult to classify fish and fish products. This publication, developed by the Food and Agriculture Organization of the United Nations (FAO), with the active support of the World Customs Organization (WCO), builds on the Harmonized System 2017 Edition. It presents all possible classifications for fish and fish products by species with a full description of each HS code, to facilitate its use within the fisheries sector.