This paper addresses emerging forms of Citizen Science (Citizen Science), and discusses their value for science, policy and society. It clarifies how the term Citizen Science is used and identifies different forms of Citizen Science. This is important, since with blurred distinctions there is a risk of both overrating and underestimating the value of Citizen Science and of misinterpreting what makes a significant contribution to scientific endeavour.The paper identifies three main forms of citizen science 1) Citizen Science as a research method, aiming for scientific output, 2) Citizen Science as public engagement, aiming to establish legitimacy for science and science policy in society, and, 3) Citizen Science as civic mobilization, aiming for legal or political influence in relation to specific issues. In terms of scientific output, the first form of Citizen Science exceeds the others in terms of scientific peer-reviewed articles. These projects build on strict protocols and rules for participation and rely on mass inclusion to secure the quality of contributions. Volunteers are invited to pursue very delimited tasks, defined by the scientists.The value of the three distinct forms of Citizen Science –for science, for policy and for society, is discussed to situate Citizen Science in relation to current policy initiatives in Europe and in the US. In quantitative terms the US, and particularly the NSF have so far taken a lead in allocating research funding to Citizen Science projects (primarily of the first form), however, the White House has recently issued a memorandum addressing societal and scientific challenges through citizen science covering all three forms discussed in this paper. As Citizen Science is currently being launched as a way to change the very landscape of science, important gaps in research are identified and policy recommendations are provided, in order for policy makers to be able to assess and anticipate the value of different forms of Citizen Science with regard to future research policy.