Policy impact assessments (IA) have become a standard procedure in all OECD member states. They are procedural instruments to assess ex-ante the intended and unintended economic, social, and environmental effects of policy to inform political decision-making. However, the consideration of environmental issues in IAs is usually weak or varies strongly, particularly in comparison to the economic dimension. Still, IAs are considered as instruments for environmental policy integration (EPI). Detailed studies if and how far IAs contribute to EPI have not been carried out, though. Against this background the PhD uses four detailed case studies to analyse IAs’ role for EPI: The IA processes on the transposition of the EU Waste Framework Directive (2008/98/EC) and the EU Biofuels Directive (2003/30/EC) are traced in Great Britain and Germany each. Their comparison is ensured by actor-centred institutionalism. The PhD project will contribute to an improved understanding of IA processes and generate lessons learnt for improving IA processes’ role for EPI.