**2. Theoretical Framework**

Despite its rather long history, sustainability as a concept is still characterized by multiple interpretations and rather blurred boundaries. [13]. The term was coined by the International Union for Conservation of Nature and Natural Resources (IUCN) in the early 1980s, and then adopted by the Brundtland Report a few years later, to indicate "development that meets the needs of the present without compromising the ability of future generations to meet their own needs" [14] (p. 54). As a result, during the last three decades, sustainable development has been at the center of the international research agenda [15]. Indeed, it has increasingly taken a central position in recent EU regional and urban politics, as evidenced for example by the decision to undertake the ESPON SUPER project. Nevertheless, although there is a "vast array of available best practices, little is known about the ways in which best practice is constructed, used, and contested, or of its implications for urban sustainability" [16] (p. 1029). Drawing on the consolidated literature on the topic, the ESPON SUPER project understands sustainability as characterized by three main aspects: temporal, thematic and institutional balance (see Figure 1). In particular, the *temporal balance* refers to the capacity to maintain long-term sustainable development for future generations and to enable the satisfaction of their needs [17]. Although often underestimated, certain factors, such as a governance quality and the durability of policies (e.g., the stability of funding), seem to effectively enhance sustainable urbanization processes [18] (p. 2). Moreover, among the characteristics that support the effectiveness of urban regeneration processes are those policies that envisage long-term sustainable targets, and which are supported by a strong political will (e.g., the UK brownfield targets, see Section 4.1.2). On the other hand, the *thematic balance* refers to three dimensions generally

referred to as the "three Es" (i.e., economy, ecology, equity) [19] or the "three Ps" (i.e., people, planet, profit) of sustainability [20]. According to the literature, any intervention faces the challenge of reconciling and enabling the coexistence of these dimensions [21]. Consequently, each of these dimensions has to be fulfilled without sacrificing the others, in order to achieve a development trajectory that is truly sustainable [22]. Finally, sustainability also depends on the *institutional balance*. For various reasons, institutional sustainability is a challenging issue for those working towards the development and implementation of sustainable urbanization policies. In fact, decisions and actions should be implemented through transparent and effective mechanisms, in line and coherent with the overall institutional framework in which they are adopted [23]. Thus, sustainable development can also be seen as a "social and political construct and, as such, the study of the operationalization of sustainable development through the implementation of specific policies provides the critical focus for research" [24] (p. 1).

**Figure 1.** Understanding sustainability (source: [25] (p. 11)).

When it comes to understanding urbanization as a phenomenon that can occur more or less sustainably, the SUPER project does not merely refer to the movement of population to cities or the expansion of the built-up area, but to all physical developments that may affect land (homes, roads, construction sites, playgrounds, airports, business parks, etc.) and to the way they are continuously influenced by policies aiming at regulating and steering development and land-use [26]. In this light, the SUPER project did not measure urbanization in Europe only in quantitative terms but also had the ambition to conceptualize it as the outcome of the countless collective and individual decisions made by humans every day about where and how they want to live, work and play within the constraints of what they can afford and what they can access. In particular, whereas urbanization patterns can be quantitatively described on the basis of key drivers like demography, economic development and society/technology (e.g., [27]), the crucial decision to convert a site from a non-urban use to an urban use is governed by the payoffs and interests of the various actors involved, which, over time, can be described as development practices. Various drivers at the macro level, including institutional and policy drivers, create (dis)incentives at the micro level to create a "local regulatory regime" or "rules of the development game" [28]. Key agents with decision-making authority, those with legal rights or economic or political clout, then interact to produce a decision on land use.

To do this, the SUPER project has designed a conceptual framework that illustrates the main cause and effect relationships that influence urbanization and land-use change mechanisms (Figure 2). The left side represents the key drivers (e.g., demography, economic development, society and technology) of urbanization and land-use patterns. In this respect, the project aimed at highlighting the drivers of change (demand) that affect land use and the institutional aspects that affect urbanization (supply). The right side of the diagram

indicates the physical outcomes of land-use decisions (which then impact on the economy, society and environment) in the different European regions, which can be measured through the use of satellite imagery and monitored over time through quantitative datasets and qualitative evidence. In order to link the drivers of urbanization and land-use and their outcomes in European regions, the research gathered and analyzed multiple examples of land use interventions throughout the European countries, in so doing aiming at opening the black box of those local practices that actually contribute to shape land-use through time.

**Figure 2.** The Sustainable Urbanization and Land-Use Practices in European Regions (SUPER\_ conceptual framework (source: [25] (p. 15).

In particular, the project acknowledges that the degree of success of any intervention aiming at steering urbanization in a specific direction is context-dependent (as certain forms of urbanization might be more sustainable than others in the different contexts) and that certain spatial governance and planning systems seem to be better equipped in relation to the promotion of sustainable urbanization than others [29]. In this light, it adopted a practice-oriented approach to explore how sustainable urbanization and land use is pursued in the different European countries and regions, thus developing a database of interventions that were explored in relation to their objectives and scopes, the types of instruments they employed, and their degree of success.
