This World Health Organization document evaluates the potential of climate-based disease early warning as a means of improving preparedness for, and response to, epidemics. The authors develop a conceptual framework for constructing and evaluating a climate-based early warning system (EWS). The report highlights that many of the most important infectious diseases, and particularly those transmitted by insects, are highly sensitive to climate variations. However, the published literature includes no full descriptions of climate-based EWS being used to influence disease control decisions.