The “lookback period” refers to how far back in time the state will look for previous DUI/OUI conviction in order to determine whether you should be treated as a repeat offender. In Rhode Island, the lookback period is five years. In Massachusetts it is your entire lifetime — for example, if you were convicted of DUI at 18 years old, had no convictions for the next 40 years, and are convicted of a second DUI at age 58, it will be considered a second offense.