As part of its responsibilities for securing civil aviation, TSA ensures that all passengers and their accessible property are screened and prohibits individuals from carrying onto aircraft items that it determines to be a threat.TSA maintains a public list of such items, known as the Prohibited Items List, and updates it as necessary. In March 2013, TSA announced it would modify the PIL to allow small knives and certain sporting equipment onto aircraft, stating the change would result in more efficient security screening. However, several aviation industry groups opposed the decision, leading TSA to reverse its decision to implement the change. GAO was asked to review TSA's procedures for modifying the PIL. This report examines, among other issues, (1) on what basis TSA modifies the PIL and the extent to which TSA assessed risk when considering recent modifications to the PIL, and (2) the extent to which TSA involved stakeholders when considering these modifications. GAO reviewed TSA's standard operating procedures, risk assessment, documentation of its decisions and stakeholder outreach, and interviewed TSA officials at six airports. This is a public version of a report with Sensitive Security Information that GAO issued in December 2014. Information TSA deemed sensitive has been redacted.