Transnational maritime crime is increasingly sophisticated, and it is expanding, both in terms of size and types of criminal activities…[t]hese crimes pose an immediate danger to people's lives and safety, they undermine human rights, hinder sustainable development and… threaten international peace and security" -Yuri Fedotov, former executive director of the United Nations Office on Drugs and Crime, 2019. The oceans provide a vast, uncontrolled arena to those engaged in illicit trade and transnational organised crime. Their activities include crimes which are maritime by nature, such as IUU fishing, as well as crimes which rely on the ocean for transport, such as drug -and wildlife trafficking. Organised criminal networks are moving these illicit commodities across the world's oceans using both commercial transport and vessels used exclusively for trafficking purposes. This chapter explores the nature of smuggling and trafficking by sea, the international legal frameworks applicable thereto and considers the case study of heroin trafficking in the Indian Ocean to illustrate how illicit trade and counteroperations work in practice.