The paper investigates the peculiarities of polycentric urban regions (PURs) in generating agglomeration economies, namely, the question of how urbanisation externalities – size, density and diversity – take effect in German PURs and influence the growth of metropolitan functions from 1995 to 2017. The metropolitan growth of PURs is compared with that of urban regions showing a dominant core. We use our findings to address questions about the forces of urbanization externalities on a regional scale. Finally, we outline what we see as emergent research topics in the interplay between metropolization and the development of PURs.

‘Polycentricity on a regional scale’, or in short the ‘polycentric region’ (PR), is a key concept in both urban and regional research and planning. But the term is used in many different ways, making it an overstretched concept unsuitable for drawing normative conclusions from. This article highlights the fact that, beyond the lowest common denominator – PRs consist of several centres (‘poly’ = many) and are situated on a regional scale (‘region’) – there is no single generally accepted definition of what a PR is. Instead, there are various historical trajectories for the formation of PRs, as well as different substantive perspectives. In this contribution, we will develop a conceptual typology and terminology describing the various dimensions of a PR for the North-Western European context on the basis of a critical literature review. The typology addresses two formation modes, four substantive perspectives and two time perspectives in an attempt to establish conceptual ‘building block(s)’ for use in empirical PR research and policy formulation. Our typology exposes knowledge gaps to be addressed in a recast PR research agenda.

The currently observed urban rescaling is characterized by two opposing trends: the regionalization of core cities and their metropolization. Such restructuring processes have consequences for a city-region's spatial and functional structure, even in traditionally multi-core metropolitan regions. This paper looks at the hypothesis that city-based business sectors belonging to the knowledge economy, dependent on their function, transaction costs and the need for geographical proximity for innovation, form different polycentric business patterns engendering either metropolization or regionalization. The aim of the paper is to analyse the metropolization and regionalization of knowledge-intensive business activities at both a municipal and a locational level, thereby shedding light on the complexity of polycentric business structures in traditional multi-core metropolitan regions. The spotlight here is on the Rhine-Ruhr metropolitan region, a prime example of a polycentric metropolitan region without a traditionally dominant metropolitan core. We show that metropolization can be seen as an agglomerating phenomenon, applying also to traditional multi-core metropolitan regions, while—with specific reference to the Rhine-Ruhr metropolitan region—“regionalization” would seem to have only a slight effect on the knowledge economy.

Growing city regions are in between the poles of high land and housing prices and national land-saving targets. The mobilization of building land to create living space and the reduction of new land take represent a conflict of goals that can only be resolved by taking an integrated look at the instruments for meeting both challenges. In order to address this conflict analytically, a regional causal loop diagram is used to map the complex mechanisms of action between housing markets and new land use for housing in core cities and their hinterland and to use it as a theoretical framework. Using a mixed-methods approach, trends in residential development as well as municipal solution strategies in German city regions are investigated. The results show increasing land use efficiency in core cities and the dense hinterland, with simultaneous high, price-induced migration gains in the less dense hinterland, where single-family housing still dominates. It becomes evident that the questions about land saving in the hinterland and housing in core cities have to be considered mutually. The regional land take can only be minimized if affordable and attractive housing is provided in the core cities, especially for families. For this, a more consistent use of existing land policy instruments in combination with a regional commitment to higher densities, especially in the hinterland, is necessary.

The paper addresses the relationship of planning systems and related processes of institutional change. It evaluates the persistence and change of the German planning system with a focus on recent impacts of Europeanisation. It shows that the ability to transform institutional patterns of spatial planning in Germany is rather limited. The German planning system has not undergone revolutionary shifts during the last five decades, but it is ‘by-passed’ by a new and mostly informal planning sphere recently. The latter is triggered by financial incentives bound to European structural funds and characterized by a strong focus on experimental spatial development.