Five scientists and engineers connected with Iran’s nuclear program have been killed or injured in recent confirmed or possible assassination attempts. It is unclear who is responsible, but the attacks raise unique policy questions about motives, effectiveness, repercussions, and legal and moral standards. Past assassination plots—including a US plan to kidnap or kill a German atomic scientist in World War II—suggest that such attempts are products of desperation: A nation tries to kill another country’s nuclear scientists when it sees no military or diplomatic options for addressing a perceived threat of existential proportions. The possible advantages of targeting another country’s nuclear scientists are modest at best, possibly delaying (but not halting) a nuclear weapons program while providing some deniability to the attacking country. The disadvantages are many, including the possibility that assassinations will inspire retaliation, reduce the likelihood of a diplomatic solution, and increase the difficulties international regulators face in monitoring a covert nuclear program. In the abstract, moral and legal strictures also weigh against such assassination efforts. As a practical matter, however, if the existential imperative is present, it will likely trump legal and ethical considerations when a nation contemplates assassinating nuclear scientists.