There is an urgent need to improve our capacity to assess the sustainability of systems relevant to development processes and the sustainability impact of policies and other drivers of change on these. Crucially, there is a clear demand from policy makers for improved sustainability assessments of policies in order to assure good governance. Although sustainability assessment of policies is practised already through a regime of integrated assessment that incorporates some sustainability impacts, assessmenttoday is partial. The scope of today's assessment tools is too limited to capture all relevant impacts and the quality of assessments is limited by gaps and weaknesses in available tools and methods. Assessment is not practised comprehensively across all relevant levels and domains of policy making and is mostly restricted to sectoral level (rather than more strategic, cross-sectoral) policy assessments. Even in this limited implementation sphere, assessment is methodologically inconsistent across applications.This paper reflects on the challenges of improving the processes and tools of sustainability assessmentand on how these challenges might be met through anew assessment paradigm of Integrated Sustainability Assessment.