The Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) states that warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice and rising global average sea level (IPCC, 2007). Projecting forward, continued warming temperatures and changes in the amount, form, and intensity of precipitation are expected, albeit with large and poorly understood regional variations. Water and watershed systems are highly climate sensitive. Climate change could thus impact the ability of water managers to sustainably meet water supply needs, comply with water quality regulations, and protect aquatic ecosystems. To reduce the likelihood of future impacts, water managers must be able to assess potential risks and opportunities, and where appropriate, implement practices and strategies to adapt to future climatic conditions. Meeting this challenge will require tools and information allowing water managers to incorporate consideration of climate change in their decision-making process. The BASINS Climate Assessment Tool (CAT) was developed in response to a widely acknowledged need for tools and information to help water managers assess and manage the impacts of climate variability and change.