Various policy options have been proposed for slowing or halting the spread of nuclear weapons, yet all rely on sound intelligence about the progress of nuclear aspirants. Historically, the United States's record of estimating foreign weapons programs has been uneven, overestimating the progress made by some proliferators while underestimating others. This paper seeks to catalogue and evaluate the intelligence work surrounding sixteen of the twenty-five states that are thought to have pursued nuclear weapons, and to provide a framework for evaluating the causes of distorted intelligence estimates of nuclear proliferation. In particular, we identify twelve specific hypotheses related to politics, culture, bureaucracy, and organizational culture, then explore how they play out in practice through two case studies (North Korea and Israel). We find that the US has overestimated nuclear programs much more frequently than it has underestimated or correctly estimated them.