The importance of metropolitan regions as national growth and development engines, and in particular as driving forces in national as well as global innovation processes is well recognized. This paper highlights the role of metropolitan regions in different contexts in order to lay a foundation for future research on metropolitan regions and their development. Specifically, the paper dwells on the role of metropolitan regions as nodes in national and international networks and as nodes of knowledge generation and innovation. Further, market potential as a concept describing the economic concentration to and the opportunities of making contacts within and between metropolitan regions is introduced. Additionally, the internal dynamic of metropolitan regions and the role of fast and slow processes is described. Lastly this paper illustrates how the input and output market potentials represent factors that adjust slowly and that play the same role for metropolitan development as metropolitan infrastructure.