Over the past decade, reanalyses of multidecadal series of past observations have become established as an important and widely utilized resource for the study of atmospheric and oceanic processes and predictability. Reanalyses, which are produced using modern versions of the data assimilation systems developed for numerical weather prediction (NWP), also are being applied increasingly in many other fields that require a record of the state of either the atmosphere or its underlying land and ocean surfaces. Estimation of renewable energy resources, calculation of microwave telecommunication signal losses, and study of bird migration are just three examples.