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The Sagan standard is the aphorism that "extraordinary claims require extraordinary evidence" (sometimes shortened to ECREE).[1] It is named for science communicator Carl Sagan, who used the phrase in his 1979 book Broca's Brain and the 1980 television program Cosmos. The standard has been described as fundamental to the scientific method and is regarded as encapsulating the basic principles of scientific skepticism.

The Sagan standard is similar to Occam's razor in that both heuristics prefer simpler explanations of a phenomenon to more complicated ones. In application, there is some ambiguity regarding when evidence is deemed sufficiently "extraordinary". The Sagan standard is often invoked to challenge data and scientific findings, or to criticize pseudoscientific claims. Some critics have argued that the standard can suppress innovation and affirm confirmation biases.

Similar statements were previously made by figures such as Thomas Jefferson in 1808, Pierre-Simon Laplace in 1814, and Théodore Flournoy in 1899. The formulation "Extraordinary claims require extraordinary proof" was used a year prior to Sagan, by scientific skeptic Marcello Truzzi. It has also been argued that philosopher David Hume first fully characterized the principles of the Sagan standard in his 1748 essay "Of Miracles".