Quackery is the promotion of fraudulent or ignorant medical practices. It is often described as "health fraud" with the salient characteristic of aggressive promotion. Psychiatrist and author Stephen Barrett of Quackwatch defines quackery as "the promotion of unsubstantiated methods that lack a scientifically plausible rationale". Common elements of general quackery include questionable diagnoses using questionable diagnostic tests, as well as untested or refuted treatments, especially for serious diseases such as cancer.

The typical quack may be a man or a woman and is likely to be striking in appearance and personality. In the past, those believed to possess special powers in healing disease included malformed persons, such as hunchbacks; rulers, such as high priests and kings; mystics; and mentally disturbed persons. The modern charlatan often boasts an alphabetical appendage, indicating the possession of academic degrees that were not conferred by any recognized scientific organization or university.

Quack advertising still relies on blatant exaggeration, use of unusual terms, creation of fear, and a promise of secrecy and financial saving. With the coming of radio, some quacks became nationally known figures, such as Norman Vincent Peale, who claimed to heal by the power of positive thinking.

In summary, quackery is the promotion of false and unproven health schemes for a profit. It is characterized by the promotion of unsubstantiated methods that lack a scientifically plausible rationale. Quacks often make exaggerated claims about their ability to heal disease, generally for financial gain.