Interscalene brachial plexus block (ISBPB) is a common nerve block given to patients who are undergoing shoulder surgery. This block has a low, but still significant, rate of complications. Reducing the volume of local anesthetic given during the block may allow some of these complications to be avoided. Participants will be randomly assigned to a group to undergo ISBPB under ultrasound or nerve stimulator guidance. Each group will initially receive 10mL of local anesthetic. The volume used for each subsequent patient depends upon the success or failure of the previous patient's block - a failure will cause the volume given to increase by 2.5mL, while a success will cause the volume to decrease by the same amount. We hypothesize that the minimum effective anesthetic volume in 50% of patients will be significantly lower in the ultrasound guided group than in the nerve stimulator group. We further hypothesize that this lower volume will lead to a decreased rate of complications.