In recent years, there has been a growing interest from researchers, academics, and architects in a new type of space generated by the invasiveness of urbanization processes, the lack of an adequate scale of regional planning, and the cycles of sprawl and shrinking. Many authors have attempted to define these spaces: Terrain Vague, Vacant Land, Urban Voids, Wasteland, or Brownfield, are just some of the terms used to define and describe abandoned, underdeveloped, and unutilized urban spaces outside the productive cycles of the city. In addition to the interest of the academic world, there is a growing emergence and multiplication worldwide of alternative projects, practices, and activities based precisely in these in-between or terrain vague spaces, which aims to both change and transform these spaces, and to preserve and enhance some of their intrinsic characteristics. In fact, by alternative projects, the authors refer to projects that both act by transforming the space and preserving the value and potential of terrain vague spaces, aiming to address the challenges of cities and achieve sustainable urban development. However, it is possible to state that there is a lack of systematic research with the aim of collecting, analyzing, and categorizing these emerging projects. The objective of this paper is the creation of a theoretical framework to identify, collect, and analyse alternative projects and practices emerging in the Terrain Vague context, and the development of the criteria to analyse for conducting a future comparative analysis, this would support the understanding of a list of differences, similarities, approaches, objectives, impacts, strategies, challenges, and difficulties. To achieve that, the paper proposes: (i) A brief theoretical introduction and definition of the concept of terrain vague inclusive of the ecological and social value; (ii) Introduction to the concepts and theoretical lenses chosen as criteria to select alternative and innovative projects, which can both enhance the values of Terrain Vague spaces and contribute to urban challenges (urban commons, everyday urbanism, temporary urbanism); (iii) Definition of the characteristics and aspects to be analysed for conducting a comparative analysis of these innovative projects (Scale/Duration, socio-ecological aims, level of participation, Existing/Flexibility).

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