This study is in 2 parts: Part 1 is a description of the anatomy of the PECS block in pain free, unoperated patients using ultrasonography (non invasive). Part 2 is the administration of the block to patients that have persistent pain after breast cancer surgery. Before and after the block we will take a detailed pain history and do neurophysiological tests called quantitative sensory testing (QST). These tests assess the patients' ability to feel warmth, cold and test their thresholds for pain. The block is then administered using ultrasound guidance and 20mls of 0.25% bupivacaine is injected in the pectoral muscles on the side with pain. Thirty minutes after the block, the pain is re assessed and the QST is repeated.