This article examines the conditions under which policy discourses can serve as contributing factors to policy change, even in the absence of changes in institutions and interests. It begins with a discussion of the role of ideas in policy analysis and how they can play a "constitutive role" as frames for policy. Drawing on a distinction between "augmentative" discourses that serve to reinforce an existing policy framework and "transformative discourses" that seek to persuade various publics of the need for significant policy change, four types of policy discourse are defined and a methodology is suggested for identifying these types. Two of these types, "challenging" and "truth-seeking," are hypothesized to be more conducive to the occurrence of significant policy change. Drawing then on case studies of policy change in Canada and Germany respectively, the article shows that a "challenging" discourse emerges in both countries, but leads to significant policy change only in Germany. Based on the comparison of the two cases, it is argued that three factors are relevant to whether a challenging discourse is successful or not: a broad consensus among core policy actors on the nature and gravity of the policy problem; the consistency of the discourse with broadly held normative values; and the persuasiveness of the "social facts" brought to bear in favour of proposed new solutions.