Implied consent laws generally require all motorists lawfully arrested for driving under the influence (DUI) to submit to chemical testing for the purpose of determining blood alcohol concentration (BAC). All states have implied consent laws on the books, though the specifics of these laws vary.
The idea behind "implied consent" laws is that all persons who opt to drive on public roadways thereby implicitly agree to take a DUI test if asked to do so by an officer. However, a motorist is required to submit to testing generally only if the requesting officer has probable cause to believe the motorist was driving under the influence.
DUI chemical testing typically involves a blood, breath, or urine test. However, blood and breath tests are most common. In most states, it's the arresting officer who selects which test the driver takes—though a handful of states let drivers choose. Testing usually takes place at the police station or a medical facility after the motorist has been arrested.
State penalties for implied consent violations vary. But across the board, motorists who refuse DUI testing are looking at license suspension. And the suspension period for a refusal is normally longer than it would be for a failed test (in other words, a BAC of .08% or more). Suspensions for chemical test refusal—as opposed to a failed test—might involve additional restrictions like having to install an ignition interlock device (IID) after the suspension. Some states also prohibit "hardship licenses" for drivers who refuse testing.
Chemical test results are often used by prosecutors to prove a DUI in criminal court. Of course, when a driver refuses testing, the prosecution won't have this evidence. Without chemical test results, it can difficult for a prosecutor to prove a "per se" DUI charge (which is based on the amount of drugs or alcohol in the driver's system). However, prosecutors are generally allowed to inform the jury of the defendant's refusal. With this information, a jury may be apt to convict even without test results.