This Product is one of 21 synthesis and assessment products (SAPs) commissioned by the U.S. Climate Change Science Program (CCSP) as part of an interagency effort to integrate federal research on climate change and to facilitate a national understanding of the critical elements of climate change. Most of these products are focused on specific substantive issues in climate science, impacts, and related topics. In contrast, the focus of this Product is methodological. Uncertainty is ubiquitous. Of course, the presence of uncertainty does not mean that people cannot act. As this Product notes, in our private lives, we decide where to go to college, what job to take, whom to marry, what home to buy, when and whether to have children, and countless other important choices, all in the face of large, and often, irreducible uncertainty. The same is true of decisions made by companies and by governments. Recent years have seen considerable progress in the development of improved methods to describe and deal with uncertainty. Progress in applying these methods has been uneven, although the field of climate science and impact assessment has done somewhat better than many others. The primary objective of this Product is to provide a tutorial for the climate analysis and decision-making communities on current best practice in describing and analyzing uncertainty in climate-related problems. While the language is largely semi-technical, much of it should also be accessible to non-expert readers who are comfortable with the treatment of technical topics at the level of journals such as Scientific American. Because the issue of how uncertainty is characterized and dealt with is of broad importance for public policy, we have also prepared a "Non-Technical Summary." Readers who lack the time or background to read the detailed Product may prefer to start there, and then sample from the main Product as they find topics they would like to learn about in greater depth.