Since the time of Trinity -- the first nuclear explosion in 1945 -- nearly 2,000 nuclear tests have been performed. Most of these occurred during the 1960s and 1970s. When the technology was new, tests were frequent and often spectacular, and they led to the development of newer, more deadly weapons. Since the 1990s, there have been efforts to limit the testing of nuclear weapons, including a U.S. moratorium and a U.N. comprehensive test ban treaty. As a result, testing has slowed -- though not halted -- and there are looming questions about who will take over for those experienced engineers who are now near retirement? Gathered here are images from the first 30 years of nuclear testing. See also "Can We Unlearn the Bomb?" and "Atomic Weapons on Film."