Dealing consistently with risk and uncertainty across the IPCC reports is a difficult challenge. Huge practical difficulties arise from the Panel's scale and interdisciplinary context, the complexity of the climate change issue and its political context. This paper reviews a few key issues on the foundations of uncertainty analysis, focusing on the objective and subjective views on uncertainty, on precise and imprecise information, and on the differences between the behavior of natural systems and human choice. It summarizes the history of the treatment of uncertainty by the IPCC, and discusses the applicability of the revised IPCC Guidance Note for the Fourth Assessment Report (AR4) in general, with special attention to costs and potentials of mitigation options. One of the key findings is that there is reason to agree to disagree: the fundamental differences between the issues covered by the IPCC's three interdisciplinary Working Groups, between the type of information available,
and between the dominant paradigms of the practitioners, legitimately lead to different approaches. We argue that properly using the Guidance, adding a pedigree analysis for key
findings, and particularly communicating the diverse nature of uncertainty to the users of the assessment would increase the quality of the assessment since it would provide information about the nature of the uncertainties in addition to their magnitude and the confidence assessors have in their findings. We make a number of concrete recommendations for improvements over earlier assessments.