Insertion of a thin, flexible tube (catheter) into the bladder to drain urine. Catheters may be used temporarily, such as during surgery and recovery, or continuously in people who cannot retain urine normally or empty their bladder. Typically, the catheter has a balloon tip that is inflated with air or sterile liquid to hold it in place after it is inserted. The balloon is then deflated to remove the catheter. The urine drains into a bag that is emptied periodically.