The maritime environment has a huge impact on the world economy and our everyday lives. The volume of maritime trade has doubled since the 1970s and reached about 90 percent of global trade in terms of volume and 70 percent in terms of value. Oceans are a shared space where marine species live and human activities (sailing, cruising, fishing, cargo transportation) are conducted. These activities generate traffic, which can lead to navigation difficulties and risks in coastal and crowded areas. The spectrum of ships goes from small sail boats to super tankers. Ships navigate using patterns that depend on their activities and objectives. These movements can conflict and can lead to collisions, posing a serious threat to the environment and human lives. Safety and security have therefore become a major concern, especially in Europe. Consideration of security by the International Maritime Organization (IMO) in the last decade has gone from ship design, education, and navigational rules (e.g. International Regulations for Preventing Collisions at Sea: COLREGS (IMO, 1972); International Convention for the Safety of Life at Sea: SOLAS (IMO, 1974)), to technical solutions for traffic monitoring. Nowadays ships are fitted out with sensors and position report systems whose objective is to identify and locate other vessels. All the information collected from these sensors can be shared on worldwide networks and stored in spatio-temporal databases. Figure 11.1 shows an example of maritime traffic collected in one day (1 April, 2012, 4M positions).