You can expunge some misdemeanors here in the State of Illinois, depending on how the matter was resolved. You must file a petition to expunge with the clerk of the county where the criminal violation occurred. You will then receive a court date and then go into court. The court will generally grant the expungment, so long as you meet the requirements. First, you cannot have been convicted of the offense. (i.e. the charge must have been dismissed or you must have received court supervision. Second, you cannot have received another conviction since.
The Illinois Appellate Defender office has the best and most comprehensive forms and explanations for expungments that I have found. Google the Illinois Appellate Defender and click on the link for expungments.