A zero-tolerance policy is one which imposes a punishment for every infraction of a stated rule. Zero-tolerance policies forbid people in positions of authority from exercising discretion or changing punishments to fit the circumstances subjectively; they are required to impose a predetermined punishment regardless of individual culpability, extenuating circumstances, or history. This predetermined punishment, whether mild or severe, is always meted out.

Zero-tolerance policies are studied in criminology and are common in both formal and informal policing systems around the world. The policies also appear in informal situations where there may be sexual harassment or Internet misuse in educational and workplace environments. In 2014, the mass incarceration in the United States based upon low-level offenses has resulted in an outcry on the use of zero tolerance in schools and communities.

Little evidence supports the claimed effectiveness of zero-tolerance policies. One underlying problem is that there are a great many reasons why people hesitate to intervene, or to report behavior they find to be unacceptable or unlawful. Zero-tolerance policies address, at best, only a few of these reasons.

According to the Online Etymology Dictionary, the first recorded use of the term "zero tolerance" was in 1972 and was originally used in US politics.

However, the term appears as early as 1939 in reference to plant diseases ("While a zero tolerance may seem a severe penalty ..."), in 1942 in reference to optical equipment ("They cut and polish glass precisely to 'zero tolerance,' ..."), and in 1945 in reference to poultry diseases ("Your safety is in buying chicks hatched from breeders showing zero tolerance."). It also appeared in the mid-1960s, in reference to an absolute ban on the pesticide heptachlor by the US Food and Drug Administration. For example, an article that appeared in the June 1963 issue of Popular Mechanics stated "Heptachlor, though, is even more toxic and has been given a 'zero tolerance' by the FDA; that is, not even the slightest trace of heptachlor is permitted on food."

The idea behind zero-tolerance policies can be traced back to the Safe and Clean Neighborhoods Act, which was approved in New Jersey in 1973 and had the same underlying assumptions. The ideas behind the 1973 New Jersey policy were later popularized in 1982, when a US cultural magazine, The Atlantic Monthly, published an article by James Q. Wilson and George L. Kelling about the broken windows theory of crime. Their name for the idea comes from the following example:

Consider a building with a few broken windows. If the windows are not repaired, the tendency is for vandals to break a few more windows. Eventually, they may even break into the building, and if it's unoccupied, perhaps become squatters or light fires inside. Or consider a sidewalk. Some litter accumulates. Soon, more litter accumulates. Eventually, people even start leaving bags of trash from take-out restaurants.

According to scholars, zero tolerance is the concept of giving carte blanche to the police for the inflexible repression of minor offenses, homeless people, and the disorders associated with them. A well-known criticism to this approach is that it redefines social problems in terms of security, it considers the poor as criminals, and it reduces crimes to only "street crimes," those committed by lower social classes and excludes white-collar crimes.