The challenge of conducting research on large and important cities is that the significance of any given city is defined very differently by the authors of various studies. This divergence of views reflects both the diverse nature of cities and also differences of approach taken in the study of cities. In this section, we provide a short overview of the different understandings of the significance of large cities by introducing the terms world city, global city, informational city, mega city, and global city-region. Of course this is only a narrow selection made from a wide range of terms used to describe cities. However, in the context of our own study on Global City Regions as Changing Sites of Governance, we consider the following definitions to be extremely valuable for developing a framework of analysis.

Research on global cities or city regions can be traced back to the world cities idea of Hall (1966) and Friedman and Wolff (1982), and also to Sassen's conception of global cities (1991).

Hall (1966) [4] defines world cities in terms of their multiple roles: “They [are] centres of political power, both national and international, and of the organizations related to government; centres of national and international trade and all kinds of economic activity, acting as entrepôts for their countries and sometimes for neighbouring countries also”.

According to Friedman and Wolff, world cities are characterized by the “predominance of financial and service sectors in the economy”. They are “closely interconnected with each other through communications and finance, [and] these regions constitute a worldwide system of control over market expansion.” [5]

Sassen (1991) defines global cities as, "cities that are strategic sites in the global economy because of their concentration of command functions and high-level producer service firms oriented to world markets; more generally cities with high levels of internationalisation in their economy and in their broader social structure" [6]. Thus, they are both centres of production and innovation as well as a home to markets.

Manuel Castells describes the new urban phenomena as the informational city, adapted to his global information society concept. The key issues within his definition are the new communication technologies and infrastructure. This includes information technology, telecommunications, air transportation, and the accordingly necessitated infrastructure.  Furthermore, he takes financial and economic performance into consideration. The informational city is to be seen as embedded in a global system of networked information flows. Within those networks, the cities are forming a hierarchy, representing nodes and hubs according to their capacities for information exchange and their interactive and innovative performance. Within the "placeless logic [of the informational economy] ... along a hierarchy of activities and functions, made up of networks and flows" the performing world city is seen as the “key millieu of innovation” [7]

The mega city concept is based on quantitative criterion assessing total population size. With respect to the quantitative definition used by the United Nations (1991), it refers to agglomerations with a minimum population size of more than 8 millions inhabitants. Other authors refer to the minimum number of 10 million. The concept has been frequently criticized for only considering the sheer size of the population, which may not guarantee any qualitative differences compared to smaller cities as regards economic performance and the role as a node and hub in a global network of cities.

The main purpose of the “Globalization and World Cities Study Group and Network” (GaWC) at Loughborough University is to produce an inventory of world cities in terms of their provision of corporate services. The basic unit of analysis in this assessment is the service firm. For any given city GaWC is looking at the presence of major corporate service firms. “By studying several firms over many cities, we are able to obtain the relative importance of a given city in terms of a particular service. By carrying out this exercise for several services, we can evaluate the overall ‘world-cityness’ of a given city.” [8]

Last but not least, a global city region is a geographic area with a city as its centre that has strong impact on the global city region’s growth and development. As Allen Scott, representative of the so called L.A. school, states, a global city region is “any metropolitan area or any contiguous set of metropolitan areas together with a surrounding hinterland of variable extent – itself a locus of scattered urban settlements – whose internal economic and political affairs are bound up in intricate ways in intensifying and far-flung extra-national relationships.” [9] The growing extension and diffusion of global cities, forming a vast arrey and a corpus of at least partly marginalized suburbanity called for the establishment of this new term. 

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