I am asked this quite a bit. The simple general answer is if the charge has been dismissed you may petition to expunge the arrest and charge. However, if you have been convicted of another criminal offense you are not eligible for an expungement. Most cases wherein you have plead guilty and successfully completed your term of supervision (NOT probation) you may petition after a certain number of years (usually two years after the last day of supervision). The process takes approximately 5 months to complete.
Procedurally the first step is filing then arguing the petition. During the hearing the judge typically wants to hear some form of remorse (even from those persons fully acquitted), how the charge either effected or could effect their professional status, and sometimes they want to hear the facts of the underlying charge as explained by both the defendant and the prosecutor. You would think that the judge would begin this type of hearing with the notion that a person who was acquitted cannot have the arrest held against them. For the most part that is true. However, there are judges who don't believe that for whatever reason. The legal tenet "innocent unless proven guilty" does not apply across the board.
Federal crimes cannot be expunged. This is due to an expungement being a state remedy, not a federal remedy. Currently there is no federal expungement or sealing statute that would allow the procedure.
There are a great number of exceptions included in the expungement statute that can affect your outcome.
Contact a qualified expungement attorney immediately for expungement assistance.