Drunk driving (or drink-driving in British English) is the act of driving under the influence of alcohol. A small increase in the blood alcohol content increases the relative risk of a motor vehicle crash.

In the United States, alcohol is involved in 32% of all traffic fatalities.

In the United States, most states have generalized their criminal offense statutes to driving under the influence (DUI). These DUI statutes generally cover intoxication by any drug, including alcohol. Such laws may also apply to operating boats, aircraft, farm machinery, horse-drawn carriages, and bicycles. Specific terms used to describe alcohol-related driving offenses include "drinking and driving", "drunk driving", and "drunken driving". Most DUI offenses are alcohol-related so the terms are used interchangeably in common language, and "drug-related DUI" is used to distinguish.

In the United Kingdom, there are two separate offences to do with alcohol and driving. The first is "Driving or attempting to drive with excess alcohol" (legal code DR10), the other is known as "In charge of a vehicle with excess alcohol" (legal code DR40) or "drunk in charge" due to the wording of the Licensing Act 1872. In relation to motor vehicles, the Road Safety Act 1967 created a narrower offense of driving (or being in charge of) a vehicle while having breath, blood, or urine alcohol levels above the prescribed limits (colloquially called "being over the limit"). These provisions were re-enacted in the Road Traffic Act 1988. A separate offense in the 1988 Act applies to bicycles. While the 1872 Act is mostly superseded, the offense of being "drunk while in charge ... of any carriage, horse, cattle, or steam engine" is still in force; "carriage" has sometimes been interpreted as including mobility scooters. )

In the European Union, the term "drink-driving" is used in the Directive (EU) 2015/413 of the European Parliament and of the Council of 11 March 2015 facilitating cross-border exchange of information on road-safety-related traffic offences. In this directive drink-driving means driving while impaired by alcohol, as defined in the law of the Member State of the offence.

Depending on the jurisdiction, a drunk driver's level of intoxication may be measured by police using three methods: blood, breath, or urine, resulting in a blood alcohol concentration, breath alcohol concentration (BrAC), or urine result. For law enforcement purposes, breath analysis using a breathalyzer is the preferred method, since results are available almost instantaneously. A measurement in excess of the specific threshold level, such as a BAC of 0.08% (8 basis points), defines the criminal offense with no need to prove impairment.

In some jurisdictions, there is an aggravated category of the offense at a higher BAC level, such as 0.12%, 0.15%, or 0.25%. In many jurisdictions, police officers can conduct field tests of suspects to look for signs of intoxication. There have been cases in Canada where officers have come upon a suspect who is unconscious after a crash and officers have taken a blood sample.^{[citation needed]}

With the advent of a scientific test for BAC, law enforcement regimes moved from field sobriety testing (e.g., asking the suspect to stand on one leg) to having more than a prescribed amount of blood alcohol content while driving. However, this does not preclude the simultaneous existence and use of the older subjective tests in which police officers measure the intoxication of the suspect by asking them to do certain activities or by examining their eyes and responses. The validity of the testing equipment/methods for determining breath and blood alcohol and mathematical relationships between breath/blood alcohol and intoxication levels have been criticized. Improper testing and equipment calibration is often used in defense of a DUI or DWI.