In response to a history of contended and ineffective policy initiatives aimed at arresting environmental problems, scientific knowledge is increasingly called for to inform decision makers in their design of better policy solutions. Based on the rationale that scientific knowledge on the environment is indispensable in environmental policy making, significant human and financial resources are being allocated to activities that are able to generate the required scientific knowledge. However, for many involved in such activities, the question arises: when do policy makers actually listen to science? This PhD thesis contributes to answering this questions; however it does this by questioning the conceptions of science that contribute to political decision-making and by exploring the relationship between scientific knowledge, other types of knowledge and policy. This PhD study employs the REDD+ (Reduced Emissions from Deforestation and forest Degradation and enhancement of forest carbon stocks) process and the phenomenon of deforestation in Zambia as research examples. The research was carried out from mid 2008 and to mid 2013 and applies a mixed methods research design. Fieldwork was carried out in Zambia during the period 2009 to 2011 at national policy level and in two field sites in the Copperbelt Province. The empirical substance consists of archival, interview, questionnaire and remote sensing data. The thesis is composed of an introductory chapter and three individual, but thematically and theoretically interlinked scientific papers. The first paper focuses on science-policy interactions seen broadly in the Zambian REDD+ process. The paper proposes five challenges, which should be addressed, among others a need for understanding the relationship between science and policy as a two-sided transaction of knowledge and for recognizing the importance of boundary work. The second paper further explores the conceptualization of science-policy interactions and the implications of boundary work as well as co-production to science-policy interaction. The paper specifically explores how knowledge related to deforestation gets translated across the social boundaries of science and policy through the interwoven processes of knowledge production, circulation, and application. It is demonstrated that production, circulation, and application of deforestation related knowledge is influenced by an epistemic community, which in a current situation of weak and contradictory empirical evidence is able to sustain a deforestation discourse centered on high forest loss and neo-Malthusian causal explanations. The third paper examines how knowing about deforestation is closely linked to issues of framing, discourse, scale, and context; and how it may be difficult to demark scientific knowledge from other types of knowledge. The paper provides suggestion on how national policy makers and policies should deal with the complex issues of knowledge and evidence in relation to deforestation and forest degradation in future REDD+ design and implementation. To curtail potential negative consequences of the identified mode of science-policy interaction in Zambia, the study concludes by making a number of proposals. The proposals are generic in nature and may be found relevant in environmental policy processes outside Zambia.