I get asked all the time, what the difference between reasonable suspicion and probable cause. Reasonable suspicion is basically a reason or belief that the crime is real. You look at it from the officers point of view. He has to point to a reasonable fact or suspicion. It has to be more than a hunch, but not full blown probable cause. Now, probable cause is when the officer has concrete evidence against you, which originally springs from probable cause. In a DUI situation, the officer would first need probable cause, and then have given the officer concrete evidence such as doing his DUI tests, that will give him probable cause to arrest you.