Police must possess probable cause to make an arrest for DUI. Judge’s can typically consider evidence of erratic driving, the defendant's appearance, the defendant's conduct, statements made by the defendant, and the defendant's performance on field sobriety tests.

Under case law, probable cause exists if an officer knows facts that would lead a reasonable person to believe that the arrestee has committed an offense. This is more than mere suspicion, or a hunch. When a police officer makes an arrest, the officer has to determine if criminal activity has occurred. The officer does not need to determine whether he or she has proof beyond a reasonable doubt. The determination of probable cause depends on the totality of the circumstances.

Field sobriety tests are admissible not only at a probable cause hearing and at trial. Sometimes, a police officer will have a Defendant take a PBT, or preliminary breath test. This test is not totally scientifically accurate. But the results may be used at a probable cause hearing.

Sometimes, a DUI arrest will involve a blood test. The results of a blood test conducted on the defendant for treatment purposes may be used by police in determining probable cause. This may seem unfair due to the doctor-patient privilege, but the Illinois legislature has stated that blood alcohol tests are not protected under the privilege.