This is a question I am often asked and it is a good one. For you to be convicted of DUI the prosecutor must prove that you were: 1) Operating a motor vehicle of some type 2) That you had consumed a substance that can impair your ability to operate the vehicle 3) That you had consumed enough of the substance to have impaired your ability to operate the vehicle There are two principle forms of evidence that a prosecutor may choose to present. One is Testimonial Evidence. This is evidence that is given by an eyewitness who must be qualified to give an opinion. This is almost always the officer that arrested you. He will relate his experiences about your appearance, behavior, odor (alcohol, etc.) and relate the results of any field sobriety tests you may have taken. The other form of evidence that will most likely be used is that of chemical test results. The most common test is the breathalyzer and some departments may administer a blood test. You have the right to refuse these, though there may be repercussions to the status of your driver's license should you choose to decline them.