Intermodal transport, the combination and integration of several modes, with the use of loading units, has been said to be more environmentally friendly than unimodal road transport for the carriage of goods. The political and scientific interest in this transport system is largely due to the sustainability and ecological aspect of the intermodal transportation system. In this paper an overview is given of studies and papers that are tackling the issue of the external effects of both intermodal and unimodal transport. An overview is given of the types of external costs that were taken into account (emissions, security, noise, etc.) and the methodologies that were used to estimate the external effects and to value these effects in terms of costs. The results of the different studies are compared and conclusions are drawn.