A whistle-blower is a person who takes a concern (such as one about safety, financial fraud, or mistreatment of research animals) outside of the organization in which the abuse or suspected abuse is occurring and with which the whistle-blower is affiliated. Not all whistle-blowing is equally adversarial to the affected organization, even though it is at least an embarrassment for an organization to be exposed as one that cannot correct its own problems.

There are many regulatory agencies, such as the U.S. Occupational Safety and Health Administration, that exist to perform oversight and to which whistle-blowers can go anonymously. Going to those charged with oversight, such as regulatory agencies, is usually seen as much less adversarial than, say, going to the media. Some people have used the term "whistler-blower" for those who raise an issue within their organization, but the more general term for a person who raises an issue inside or outside an organization is "complainant."