The object of analysis of the transformation of socio-technical regimes towards sustainability requires an understanding of the role of innovation in niches, and especially the role of grassroots innovations. The normative dimension of sustainability implies a definition of innovation for sustainability or sustainable innovation. As we will see, there are two paths to define sustainable innovation one from the research fields and one from the business world (ISO, certification schemes et similia). Both the perspective are essential to position grassroots innovation in the area of sustainable innovation as a special form of social innovation. In the following paragraphs, we will first introduce the two approaches for sustainable innovation (theoretical and managerial), then definition and characteristics of social innovation as a specific form of the previous. Finally, the concept of grassroots innovation, as a special kind of sustainable and social innovation, will be introduced. Its characteristics and operational framework will then be referred to as the general framework for the analysis of the sport grassroots initiatives presented in the second part of the book.