The global application of impact assessment instruments to achieve a variety of policy integration goals (e.g. the mainstreaming of environmental, gender or economic efficiency concerns) continues to proliferate. These instruments represent important components of contemporary political governance and hence are an important locus for applied research. This special issue of Impact Assessment and Project Appraisal critically examines 'state-of-the-art' knowledge and understanding of the effectiveness of impact assessment instruments. Six articles explore this subject from a variety of orientations (in terms of theoretical versus empirical emphasis, policy integration concerns, contributors' beliefs and framing etc.). Individually and cumulatively, these articles make a powerful contribution to learning about the 'thorny' issue of effectiveness and its implications for the theory and practice of impact assessment.