This report examines the development and use of scenarios in global climate change applications. It considers scenarios of various types - including but not limited to emissions scenarios - and reviews how they have been developed, what uses they have served, what consistent challenges they have faced, what controversies they have raised, and how their development and use might be made more effective. The report is Synthesis & Assessment Product 2.1b of the US Climate Change Science Program. By synthesizing available literature and critically reviewing past experience, the report seeks to assist those who may be conducting, using, or commissioning scenarios related to global climate change. Scenarios are used to support planning and decision-making when issues have deep or poorly characterized uncertainty and high stakes, often accompanied by long time horizons. These conditions apply to the major decisions about how to respond to global climate change. As scientific research advances our knowledge of the climate's present state and trends, its patterns of variability, and its responses to external forcings, we are gaining an increasingly clear view of risks that may be realized late this century or beyond. These future risks are linked to near-term socio-economic trends and decisions in both public and private sectors. Some near-term decisions - such as investment in long-lived capital equipment, new resources, or new technologies in the energy sector - can influence long-term trends in the emissions contributing to climate change. Other near-term decisions - such as investment in water resources infrastructure or coastal development - can influence how adaptable and how vulnerable future society will be to the impacts of climate change.