Multimodal transport, that is using two or more transport modes for a trip between which a transfer is necessary, seems an interesting approach to solving today's transportation problems with respect to the deteriorating accessibility of city centres, recurrent congestion, and environmental impact. Combining private transport and public transport in a truly multimodal transport system offers opportunities to capitalise on the strengths of the various systems while avoiding their weaknesses. The requirements for such a multimodal transport system, however, are high. Travellers have to be aware of the possibilities to change modes and the related benefit. Thus high quality travel information is crucial. Transfers between transport modes and services should be seamless, setting new standards for the design of transfer nodes and for the synchronisation of time-tabled transport services. Multimodal transport requires new organisational and financial arrangements between all actors involved. The most fundamental component of a multimodal transport system, however, is the multimodal transport network that consists of networks for private transport, public transport, and other transport services that are part of the multimodal transport system, including of course the transfer possibilities between these networks. This thesis investigates the consequences of multimodal travelling for designing multimodal transport networks. It describes the characteristics of multimodal travel today and assesses its future potential. The analysis focuses on the way transport networks are organised in hierarchical network structures and determines the main mechanisms leading to these hierarchical network structures. Furthermore, an analysis is made of the role in a multimodal transport system of transport services other than private transport or public transport. The results provide new insights into the mechanisms determining hierarchical transport network structures. They show the potential impact of multimodal transport especially on the capacity requirements for public transport, and they show the possible roles of the various transport services that may be part of a multimodal transport system.