Assessing the state of U.S. nuclear weapons capabilities presents several challenges. First, the U.S. has elected to maintain the weapons—based on designs from the 1970s—that were in the stockpile when the Cold War ended rather than develop new weapons. Second, detailed data about the readiness of nuclear forces, their capabilities, and weapon reliability are not publicly available, and this makes analysis difficult. Third, the U.S. nuclear enterprise is comprised of many components, some of which are also involved in supporting conventional missions. For example, bombers do not fly with nuclear weapons today as they routinely did during the Cold War (although they are capable of doing so again if the decision should ever be made to resume this practice). Also, the U.S. National Nuclear Laboratories perform a variety of functions related to nuclear nonproliferation, medical research, and nuclear detection, among many others, as opposed to focusing solely on the nuclear weapons mission.