This is a list of notable conspiracy theories. Many conspiracy theories relate to supposed clandestine government plans and elaborate murder plots. They usually deny consensus opinion and cannot be proven using historical or scientific methods, and are not to be confused with research concerning verified conspiracies, such as Germany's pretense for invading Poland in World War II.

In principle, conspiracy theories might not always be false, and their validity depends on evidence as for any theory. However, they are often implausible prima facie due to their convoluted and all-encompassing nature. Conspiracy theories tend to be internally consistent and correlate with each other; they are generally designed to resist falsification either by evidence against them or a lack of evidence for them.

Psychologists sometimes attribute proclivities toward conspiracy theories to a number of psychopathological conditions such as paranoia, schizotypy, narcissism, and insecure attachment, or to a form of cognitive bias called "illusory pattern perception". However, the current scientific consensus holds that most conspiracy theorists are not pathological, but merely exaggerate certain cognitive tendencies that are universal in the human brain and probably have deep evolutionary origins, such as natural inclinations towards anxiety and agent detection.