Transportation modes are essential components of transport systems since they are the means of supporting mobility. Modes can be grouped into three broad categories based on the medium they exploit: land, water, and air. Each mode has its own requirements and features and is adapted to serve specific freight and passenger traffic demands. This gives rise to marked differences in how the modes are deployed and utilized in different parts of the world. More recently, there has been a trend towards integrating the modes through intermodality and linking the modes ever more closely into production and distribution activities. At the same time, however, passenger and freight activity is becoming increasingly separated across most modes.