For the aircraft, see A-5 Vigilante.

In modern terms, vigilantes are militias or police which attempt law enforcement, in the usual phrase, "by taking the law into their own hands". Vigilantes often operate in secret.

In the United States vigilantism was especially strong in Montana during the days of the gold rush and in California during the struggle for control of the gold fields after the gold rush of 1849 and during labor unrest between the World Wars. However vigilantism frequently broke out in other areas of the country and often resulted in lynching of suspected criminals. Not all activity was spur of the moment. Nineteenth century America had several well-organized vigilance committees.

In Spanish, vigilante means 'watchman', which is how many of these groups started, where there was no organized law enforcement.

Vigilantism is also the basic concept from which much contemporary fiction is derived, including stories published in dime novels and comic books. Many of the heroes of pulp fiction, such as Doc Savage and The Shadow, and comic book superheroes, such as Superman and Spider-Man, are considered vigilantes due to their actions being self-motivated, and generally aimed at a good result, rather than being duly-appointed officers of the law. The extreme of the vigilante-type superhero is Marvel Comics' character The Punisher, who used flagrantly illegal methods such as assassination to fight criminals.

In the modern day western world, vigilantism often occurs when the local population is frustrated with the complex and seemingly unfair court procedures and rulings that apparently allow felons to walk free or be found not guilty. Vigilante "justice" often takes the form of assault, arson, and possibly even death of the accused person. At times, certain criminals will opt to forgo parole for fear of receiving vigilante justice once released.