Maritime transport flows are the lifeblood of global trade. They are also the dominant means of transporting a range of potentially destabilizing commodities that threaten states and societies throughout the developing and developed worlds. Maritime transport is the preferred mode for this clandestine trade not least because the high seas are the most difficult areas of our planet to monitor or regulate. No state effectively controls the vastness of this territory; despite advances in the application of satellite and ship monitoring technologies and enhanced information sharing in certain areas, much remains unknown. This SIPRI Policy Paper aims to fill a crucial knowledge gap by, for the first time in a public document, providing an analysis of the ships involved in the transport of some of the most destabilizing commodities: narcotics, arms and dual-use goods essential to the development of weapons of mass destruction. It also identifies 'choke points' and weaknesses in trafficking techniques, which, if the political will and requisite resources exist, can be exploited to better prevent destabilizing maritime transfers. The study further recommends actions that could be implemented at relatively low cost, using existing mechanisms, but that could have major and lasting benefits for global economic and human security. Through this study, the authors offer a much-needed focus on and practical solutions to one of the most important global security challenges for policymakers, civil society and industry in the 21st century.