DUI (driving under the influence), DWI (driving while intoxicated), and OWI (operating while intoxicated) are similar criminal offenses that involve the operation of a motor vehicle, electric vehicle, or sometimes an otherwise-powered vehicle—including boats, aircraft, and bicycles—by an operator who is under the influence of or intoxicated by alcohol or drugs. Per se (pronounced purr-say) intoxication is intoxication by definition—as defined in the state’s statutes by blood alcohol concentration (BAC) of .08 or more (.05 in Utah). And even if a person is not per se intoxicated, they may be intoxicated by not having the normal use of their mental or physical faculties (abilities), for example, and may be charged with a criminal offense.

In some contexts the terms DUI and DWI are used interchangeably, with some states using one term and other states using the other term to define what is essentially the same criminal offense. But in some states both criminal charges are available and there is a legal distinction between them.

For example, in some states with both criminal offenses, DUI is the criminal offense of operating a vehicle under the influence of prescription or recreational drugs, and DWI is the offense of operating a vehicle while intoxicated by alcohol. In some states DWI refers to per se intoxication when the driver’s BAC is above the threshold defined by statute—.08 (.05 in Utah)—and DUI is a criminal offense charged when the police officer believes the driver is under the influence of alcohol, despite the driver’s BAC being below the statutory threshold of .08. And in some states DUI is a separate offense for drivers under the legal age of drinking (21) who have some alcohol in their system, but less than a .08 BAC.

These and other similar alcohol-related operating offenses—including aggravated or enhanced forms of these offenses—vary from state to state in their definitions and potential punishments and are generally located in a state’s statutes—often in the penal code or criminal code.

In Montana (MT), the terms DUI (driving under the influence), DWI (driving while intoxicated), and OWI (operating while intoxicated) refer to the criminal offense of operating a motor vehicle while impaired by alcohol or drugs. Montana law primarily uses the term DUI. The state's legal limit for blood alcohol concentration (BAC) is .08, in line with federal standards and most other states. A driver can be charged with DUI if their BAC is at or above this limit (per se intoxication). However, a driver can also be charged with DUI if they are found to be impaired due to alcohol or drugs, even if their BAC is below .08, based on the observation of their mental or physical faculties. For drivers under the age of 21, Montana has a 'zero tolerance' law, meaning any detectable amount of alcohol can result in a DUI charge. Penalties for DUI in Montana can include fines, license suspension, and possible jail time, with increased severity for repeat offenses or higher BAC levels.