Scientific skepticism (also spelled scepticism) is the practice of questioning the veracity of claims lacking empirical evidence or reproducibility, as part of a methodological norm pursuing "the extension of certified knowledge". For example, Robert K. Merton asserts that all ideas must be tested and are subject to rigorous, structured community scrutiny (see Mertonian norms).
This sort of skepticism is also called rational skepticism, and it is sometimes referred to as skeptical inquiry.
Scientific skepticism is different from philosophical skepticism, which questions our ability to claim any knowledge about the nature of the world and how we perceive it. Scientific skepticism primarily uses deductive arguments to evaluate claims which lack a suitable evidential basis. The New Skepticism described by Paul Kurtz is scientific skepticism.
Scientific skeptics believe that empirical investigation of reality leads to the truth, and that the scientific method is best suited to this purpose. Considering the rigor of the scientific method, science itself may simply be thought of as an organized form of skepticism. This does not mean that the scientific skeptic is necessarily a scientist who conducts live experiments (though this may be the case), but that the skeptic generally accepts claims that are in his/her view likely to be true based on testable hypotheses and critical thinking.
Scientific skeptics attempt to evaluate claims based on verifiability and falsifiability and discourage accepting claims on faith or anecdotal evidence. Skeptics often focus their criticism on claims they consider to be 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 claims of paranormal or anomalous phenomena should be critically examined 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, theories are judged on many criteria, such as falsifiability, Occam's Razor, and explanatory power, as well as the degree to which their predictions match experimental results. Skepticism is part of the scientific method; for instance an experimental result is not regarded as established until it can be shown to be repeatable independently.
By the principles of skepticism, the ideal case is that every individual could make his own mind up on the basis of the evidence rather than appealing to some authority, skeptical or otherwise. In practice this becomes difficult because of the amount of knowledge now possessed by science, and so an ability to balance critical thinking with an appreciation for consensus amongst the most relevant scientists becomes vital.
Not all fringe science is pseudoscience. For instance, some proponents of repressed memories apply the scientific method carefully, and have even found some empirical support for their validity,   though the theories have not received complete scientific consensus.   
Empirical or scientific skeptics do not profess philosophical skepticism. Whereas a philosophical skeptic may deny the very existence of knowledge, an empirical skeptic merely seeks likely proof before accepting that knowledge.
See main article: List of topics characterized as pseudoscience.
Some of the topics that scientifically skeptical literature questions include health claims surrounding certain foods, procedures, and alternative medicines; the plausibility and existence of supernatural abilities (e.g. tarot reading) or entities (e.g. poltergeists, angels, gods - including Zeus); the monsters of cryptozoology (e.g. the Loch Ness monster); as well as creationism/intelligent design, dowsing, conspiracy theories, and other claims the skeptic sees as unlikely to be true on scientific grounds. 
Skeptics such as James Randi have become famous for debunking claims related to some of these. Paranormal investigator Joe Nickell cautions, however, that "debunkers" must be careful to engage paranormal claims seriously and without bias. He explains that open minded investigation is more likely to teach and change minds than debunking. Many skeptics are atheists or agnostics, and have a naturalistic world-view; however, some committed skeptics of pseudoscience including Martin Gardner have expressed belief in a god.
Richard Cameron Wilson, in an article in New Statesman, wrote that some advocates of discredited intellectual positions such as AIDS denial and Holocaust denial engage in pseudoskeptical behavior when they characterize themselves as "skeptics" despite cherry picking evidence that conforms to a pre-existing belief. According to Wilson, who highlights the phenomenon in his book Don't Get Fooled Again (2008), the characteristic feature of false skepticism is that it "centres not on an impartial search for the truth, but on the defence of a preconceived ideological position".
Scientific skepticism is itself sometimes criticized on this ground. The term pseudoskepticism has found occasional use in controversial fields where opposition from scientific skeptics is strong. For example, in 1994, Susan Blackmore, a parapsychologist who became more skeptical and eventually became a CSICOP fellow in 1991, described what she termed the "worst kind of pseudoskepticism":
Commenting on the labels "dogmatic" and "pathological" that the "Association for Skeptical Investigation" puts on critics of paranormal investigations, Robert Todd Carroll of the Skeptic's Dictionary argues that that association "is a group of pseudo-skeptical paranormal investigators and supporters who do not appreciate criticism of paranormal studies by truly genuine skeptics and critical thinkers. The only skepticism this group promotes is skepticism of critics and [their] criticisms of paranormal studies."
In practice, the term is most commonly applied to the examination of fringe and pseudoscientific claims and theories which appear to be outside mainstream science and medicine, rather than to the routine discussions and challenges among scientists.
See also: Anti-cult movement.
Skepticism is an approach to strange or unusual claims where doubt is preferred to belief, given a lack of conclusive evidence. Skeptics generally consider beliefs in the extraterrestrial hypothesis (ETH) and psychic powers as misguided, since no empirical evidence exists supporting such phenomena. The Ancient Greek philosopher Plato believed that to release another person from ignorance despite their initial resistance is a great and noble thing. Modern skeptical writers address this question in a variety of ways.
Bertrand Russell argued that individual actions are based upon the beliefs of the person acting, and if the beliefs are unsupported by evidence, then such beliefs can lead to destructive actions. James Randi also often writes on the issue of fraud by psychics and faith healers. Critics of alternative medicine often point to bad advice given by unqualified practitioners, leading to serious injury or death. Richard Dawkins points to religion as a source of violence (notably in his book, The God Delusion), and considers creationism a threat to biology. Some skeptics, such as the members of The Skeptics' Guide to the Universe podcast, oppose certain cults and new religious movements because of their concern about what they consider false miracles performed or endorsed by the leadership of the group. They often criticize belief systems which they believe to be idiosyncratic, bizarre or irrational.
See also: List of skeptics and skeptical organizations.
. James Randi. Flim-Flam! Psychics, ESP, Unicorns, and Other Delusions. Prometheus Books. June 1982. 342. 0-345-40946-9.
. James Randi. Arthur C. Clarke. An Encyclopedia of Claims, Frauds, and Hoaxes of the Occult and Supernatural. St. Martin's Griffin. 1997. 336. 0-312-15119-5.
. Carl Sagan. Ann Druyan. The Demon-Haunted World: Science as a Candle in the Dark. Ballantine Books. 1997. 349. 0-345-40946-9.
. Martin Gardner. Fads and Fallacies in the Name of Science. Dover Publications. 1957. 373. 0-486-20394-8.
. Michael Shermer. Why People Believe Weird Things. St Martins Griffin and Company. 1997. 349. 978-0-8050-7089-7.
. Carl Sagan. Contact. Orbit. 1997. 432. 1-85723-580-0.
. Carl Sagan. Billions and Billions: Thoughts on Life and Death at the Brink of the Millennium. Ballantine Books. 1998. 320. 0-345-37918-7.
. Paul Kurtz. The New Skepticism: Inquiry and Reliable Knowledge. Prometheus Books. 1992. 371. 0-87975-766-3.
. Michael Langone. Recovery from Cults: Help for Victims of Psychological and Spiritual Abuse. W. Norton. American Family Foundation. June 1995. 432. 0-393-31321-2.