<p>The security of the U.S. commercial aviation system has been a growing concern since the 1970's when the hijacking of aircraft became a serious problem. Over that period, federal aviation officials have been searching for more effective ways for non-invasive screening of passengers, luggage, and cargo to detect concealed explosives and weapons. To assist in this effort, the Transportation Security Administration (TSA) asked the NRC for a study of emerging screening technologies. This book - the fourth of four - focuses on data fusion as a means to significantly improve the ability of the existing suite of airport detection systems and access control systems to detect and prevent attacks. The book presents a discussion of the data fusion, an analysis of current data fusion efforts, and an assessment of data fusion opportunities for various airport security activities.</p>