The primary goal of the annual State of the Climate collection of articles is to document the weather and climate events in 2009 from around the world and put them into accurate historical perspective, with a particular focus on unusual or anomalous events. In total, 37 Essential Climate Variables were reported in 2009. This State of the Climate report brings together more than 300 authors from every continent and from over 160 different research groups to collaborate, share data and insights, and describe the observed changes in climate from different perspectives.