Nuclear espionage is sharing a country's secrets about nuclear weapons to other countries without permission. Since nuclear weapons have been invented, there have been many cases of known nuclear espionage, and also many cases where it has thought to have happened, but has not been proven. Because nuclear weapons are usually considered the most important of state secrets, all nations with nuclear weapons have strict rules against sharing information about how nuclear weapons are designed, where they are kept, and other things about them. There are also rules that keep countries who have signed non-proliferation agreements (which are meant to stop the spread of nuclear weapons) from sharing information about such weapons with the public.