3 The global cargo ship network comprehensive data of real ship journeys allowing us to construct the actual network. We show that it has a small-world topology where the combined cargo capacity of ships calling at a given port (measured in gross tonnage) follows a heavy-tailed distribution. This capac- ity scales superlinearly with the number of directly connected ports. We identify the most central ports in the network and find several groups of highly interconnected ports showing the importance of regional geopolitical and trading blocks. A high-level description of the complete network, however, does not yet fully capture the network’s complexity. Unlike previously studied transportation networks, the GCSN has a multi-layered structure. There are, broadly speaking, three classes of cargo ships – container ships, bulk dry carriers, and oil tankers – that span distinct subnetworks. Ships in different categories tend to call at different ports and travel in distinct patterns. We analyze the trajectories of individual ships in the GCSN and develop techniques to extract quantitative information about characteristic movement types. With these methods we can quantify that container ships sail along more predictable, frequently repeating routes than oil tankers or bulk dry carriers. We compare the empirical data with theoretical traffic flows calculated by the gravity model. Simulation results, based on the full GCSN data or the gravity model differ significantly in a population-dynamic model for the spread of invasive species between the world’s ports. Predictions based on the real network are thus more informative for international policy decisions concerning the stability of worldwide trade and for reducing the risks of bioinvasion. An analysis of global ship movements requires detailed knowledge of ships’ arrival and departure times at their ports of call. Such data have become available in recent years. Starting in 2001, ships and ports have begun installing Automatic Identification System (AIS) equipment. AIS transmitters on board of the ships automatically report the arrival and departure times to the port authorities. This technology is primarily used to avoid collisions and increase port security, but arrival and departure records are also made available by Lloyd’s Register Fairplay for commercial purposes as part of its Sea-web data base (www.sea- web.com). AIS devices have not been installed in all ships and ports yet, and therefore there are some gaps in the data. Still, all major ports and the largest ships are included, thus the II. DATA