Scientific skepticism or rational skepticism (also spelled scepticism), sometimes referred to as skeptical inquiry, is a position in which one questions the veracity of claims lacking scientific evidence. In practice, the term most commonly refers to the examination of claims and theories that appear to be unscientific, rather than the routine discussions and challenges among scientists. Scientific skepticism differs from philosophical skepticism, which questions humans' ability to claim any knowledge about the nature of the world and how they perceive it, and the similar but distinct methodological skepticism, which is a systematic process of being skeptical about (or doubting) the truth of one's beliefs.

The skeptical movement (British spelling: sceptical movement) is a contemporary social movement based on the idea of scientific skepticism. The movement has the goal of investigating claims made on fringe topics and determining whether they are supported by empirical research and are reproducible, as part of a methodological norm pursuing "the extension of certified knowledge".

Roots of the movement date at least from the 19th century, when people started publicly raising questions regarding the unquestioned acceptance of claims about spiritism, of various widely held superstitions, and of pseudoscience. Publications such as those of the Dutch Vereniging tegen de Kwakzalverij (1881) also targeted medical quackery. Using as a template the Belgian organization founded in 1949, Comité Para, Americans Paul Kurtz and Marcello Truzzi founded the Committee for the Scientific Investigation of Claims of the Paranormal (CSICOP), in Amherst, New York, in 1976. Now known as the Committee for Skeptical Inquiry (CSI), this organization has inspired others to form similar groups worldwide.

Scientific skeptics maintain that empirical investigation of reality leads to the most reliable empirical knowledge, and suggest that the scientific method is best suited to verifying results. Scientific skeptics attempt to evaluate claims based on verifiability and falsifiability; they discourage accepting claims which rely on faith or anecdotal evidence.

Paul Kurtz described scientific skepticism in his 1992 book The New Skepticism, calling it an essential part of scientific inquiry. The Skeptics Society describes it as "the application of reason to any and all ideas—no sacred cows allowed." Robert K. Merton introduced Mertonian norms, which assert that all ideas must be tested and are subject to rigorous, structured community scrutiny. Kendrick Frazier said that scientific skeptics have a commitment to science, reason, evidence, and the quest for truth. Carl Sagan emphasized the importance of being able to ask skeptical questions, recognizing fallacious or fraudulent arguments, and considering the validity of an argument rather than simply whether we like the conclusion. Similarly, Steven Novella described skepticism as selecting "beliefs and conclusions that are reliable and valid to ones that are comforting or convenient" and as the study of "pitfalls of human reason and the mechanisms of deception so as to avoid being deceived by others or themselves". Brian Dunning called skepticism "the process of finding a supported conclusion, not the justification of a preconceived conclusion.''

Skeptics often focus their criticism on claims they consider implausible, dubious or clearly contradictory to generally accepted science. Scientific skeptics do not assert that unusual claims should be automatically rejected out of hand on a priori grounds—rather they argue that one should critically examine claims of paranormal or anomalous phenomena and that extraordinary claims would require extraordinary evidence in their favor before they could be accepted as having validity. From a scientific point of view, skeptics judge ideas on many criteria, including falsifiability, Occam's Razor, Morgan's Canon and explanatory power, as well as the degree to which their predictions match experimental results.^{[need quotation to verify]}

Skepticism in general may be deemed part of the scientific method; for instance an experimental result is not regarded as established until it can be shown to be repeatable independently.

The Sci.Skeptic FAQ characterizes the skeptic spectrum as divided into "wet" and "dry" skeptics, primarily based on the level of engagement with those promoting claims that appear to be pseudoscience; the dry skeptics preferring to debunk and ridicule, in order to avoid giving attention and thus credence to the promoters, and the "wet" skeptics, preferring slower and more considered engagement, in order to avoid appearing sloppy and ill-considered and thus similar to the groups all skeptics opposed.