Transitions, or socio-technological transformations, towards sustainability can be considered as a long-term transformation at the level of society as a whole, which in turn consists of a sequence of short-term innovations. The direction and speed of transitions are largely determined by the collective innovation decisions of various actors. A crucial characteristic of transitions is that they involve many uncertainties. The uncertainties that actors perceive greatly influence their innovation decisions. Namely, perceived uncertainties might stimulate some actors to fulfill certain key activities that are crucial for achieving a transition, while blocking other actors from undertaking these activities. In order to understand and manage transitions, insight into the types of perceived uncertainties that dominate the innovation decisions is essential. Furthermore, we need to understand if perceived uncertainties block or stimulate transitions by analyzing how actors respond to perceived uncertainties.This article focuses on the first transition phase: the pre-development phase. An interesting case to study the role of uncertainties in this phase is the introduction of micro-CHP in the Netherlands. The main questions of this article are: Which types of perceived uncertainties are dominant for the innovation decisions related to micro-CHP in the Netherlands? How do actors react to the perceived uncertainties?The case results demonstrate that different types of uncertainties influence the innovation decisions of the involved actors. The most dominant sources of uncertainty are technological and political uncertainty. Furthermore, the case shows that responses to uncertainty vary largely between various types of actors. Perceived uncertainties seem to block some actors, but induce other actors in fulfilling key activities that are essential for achieving a transition. We discuss how these insights can be used for improving policy for stimulating transitions.