Drunk driving laws are intended to prevent the operation of a powerful and potentially dangerous machine when the operator cannot be in adequate control. Intoxication is shown in one of two ways:

(1) a blood alcohol level in excess of a certain amount, or
(2) proving that the driver or operator was impaired from the use of alcohol or illegal drugs.

The first method is the method preferred by prosecutors. The proof does not rely on anyone's observation or judgment of someone's behavior, but depends solely on the results of a blood alcohol test. Laws often require a person who is suspected of driving while drunk or using illegal drugs to give a sample of his or her blood or breath for chemical testing purposes. These laws are known as "implied consent" laws, because they provide that by operating a motor vehicle, the driver has given his or her consent to such a test. When a sample is taken, it is analyzed by a machine to determine the concentration of alcohol in a person's blood. The maximum blood alcohol level varies from state to state. For many years, the most common maximum level was .10 percent, but most states have now lowered the level to .08. Any driver or operator who has a blood alcohol level over the legal limit is considered legally intoxicated. The results of the test are usually considered conclusive, and can be challenged only by showing that the test failed for a reason such as faulty or malfunctioning test equipment, improper sampling, faulty preservation of the sample, or (in the case of a breath test) a foreign object in the mouth when the test was conducted.

Impairment may also be proven by the facts and circumstances surrounding the incident for which the driver was stopped or arrested. These facts and circumstances include observations of the driver by eyewitnesses, statements made by the driver or operator, and circumstantial evidence (for example, evidence that a defendant left a bar after being inside for several hours). The inquiry focuses on whether the defendant's ability to drive was impaired. Law enforcement officers have a number of standard tests for impairment, done at the time a driver is stopped, known as "field sobriety tests." These tests include walking a straight line by placing one foot directly in front of the other, holding one's arms straight out at the sides and touching the nose with closed eyes, counting backwards, and reciting the alphabet. Other evidence of impairment may include a law enforcement officer's observation of the defendant's driving, which probably was the reason the driver was stopped in the first place. Conduct such as driving too fast or too slowly, weaving across lanes, and going through stop signs or stoplights may be considered evidence of a driver's impairment. Drivers often will tell an officer who stops them that they have been drinking, how much they've had to drink, and when they had it. Such statements may also be evidence of impairment.