This chapter addresses sources with materials and the related methodology to be employed in order to address two key questions (Sect. 2.1). Consequently, the term sustainability is presented as an abstract, ephemeral and evolving omnipresent expression which is to be studied through various branches of social sciences based on heterogenous data via predominantly qualitative methods and mechanisms. The first question is about the search for the foundation, development and meaning of the concept of sustainability (Sect. 2.2). The second question is about the search for the regime and duties related to such an evolving concept (Sect. 2.3). This inevitably leads to the revelation of challenging tensions between various approaches, in particular between expectations and reality, and to the need to wrap it up by tying together the ends (Sect. 2.4).