The first nuclear weapon was tested in Alamogordo, NM, in July 1945. In the following month, Hiroshima and Nagasaki were destroyed through the explosion of two nuclear warheads. These horrifying strikes directly led to the surrender of Japan almost 4 years after its attack on Pearl Harbor. An industrial scale effort with more than 130,000 employees produced the first nuclear fission weapons during World War II. With the United States and its allies facing totalitarian aggressors in the European and Pacific theaters, many elite scientists, engineers, and technicians supported the Manhattan Project through their scientific and technological innovations. Now, 72 years later, despite enormous international efforts to limit nuclear weapons technology to the initial Cold War nuclear powers, knowledge and technology have further proliferated, and today nine countries possess nuclear weapons. Most recently the impoverished Democratic People's Republic of Korea has built and tested a small number of nuclear warheads relying on modest resources in capital and talent. The aphorism "today's sensation is tomorrow's calibration" fittingly describes the rapid progress in science and technology. In the context of nuclear weapons, the fast technological progress raises concerns that nuclear weapons will become reachable one day for non-state actors. This lecture will summarize the status of present nuclear arsenals and technology. It will discuss the possibility of nuclear weapons materials and technologies falling into the hands of terrorists. The lecture will describe the consequences of a possible nuclear attack on a major population center in the United States and review possible measures that can reduce the risk of nuclear terrorism.