Participants undergoing the Merci Retriever or Penumbra System procedure will have a cerebral arteriogram with pictures taken with dye prior to the procedure to determine the location of the blockage, and following the procedure to determine if blood supply has been restored. The total mechanical embolectomy procedure with either device will take approximately 1 to 2 hours. Participants will have brief neurological exams several times during this time to monitor changes in their neurological condition.