I’d like to thank the Emilio Segrè Visual Archives; the American Institute of Physics; Mary Straka, science librarian at the Argonne Research Library, Argonne National Laboratory; the Smithsonian Institution Archives; Allison Rein, at the Niels Bohr Library & Archives; Alexandra Levy, at the Atomic Heritage Foundation; the Los Alamos Historical Archives; and Brown University Department of Mathematics. At Little, Brown Books for Young Readers, my wonderful editors Alvina Ling and Samantha Gentry, and the entire staff. And, as always, my thanks go to the wonderfully talented Rob Weisbach, agent and friend extraordinaire. Gratitude to my mother, Celeste, and my sister, Francesca, for listening to me rattle on about atomic energy and female scientists and the joys of writing.