A prosecutor becomes involved in a criminal case in one of two ways: referral from the police who have investigated, arrested, searched, and processed an alleged offender; or in some states through a preliminary hearing or grand jury proceeding. Drunk driving cases generally make their way to the prosecutor via the police-the first method described. In making the decision to go forward with a case, the prosecutor considers three things: whether the case is legally sound, whether it can be proved, and the relevant policy considerations. If the prosecutor decides not to go forward with a case, the case will be over, no matter how much any alleged victim or the police, or even the court, may want the alleged offender prosecuted.

The prosecutor must be assured that there is enough reliable evidence to prove the DUI drunk driving charge before he or she will bring the case to trial. Policy considerations are always part of the decision to prosecute a particular defendant, too, because the prosecutor's job is to serve justice in the public interest, not just win every possible case. The defendant might have mental or physical problems that make pre-trial diversion, like alcohol or drug treatment or a suspended prosecution, a better option than trial.