The Introduction lays down the foundations to build a theoretical frame in which citizen sensing for environmental risk monitoring will be discussed in light of selected doctrinal discussions from risk governance doctrine. The chapter essentially is a problem statement as it targets citizen sensing in relation to the issue of a perceived legitimacy gap in current (environmental) risk governance. Developing the sociological argument that citizen sensing may respond to a legitimacy crisis in risk governance, the Introduction can be considered the foundation for the research developed in the next chapters. Prior to exploring a potential contribution of citizen sensing to risk governance, it is worth to inquire whether current institutional risk governance needs adaptations. Furthermore, the chapter engages in a reflection on the legitimacy of a possible contribution from citizen sensing initiatives.