A new science. The beginnings. The setting ; Rays and radiation ; Becquerel's discovery -- The Curies. Maria Skłodowska ; A consequential meeting ; New elements! -- Rutherford, Soddy, particles, and alchemy? Rutherford and the rays ; Where did the energy come from? ; Material rays? Discovery of the beta particle ; Thorium's rays ; Vanishing radioactivity ; Transmutation! ; A missed discovery ; Reactions ; Atomic energy? ; Tragedy ; More rays ; The alpha particle -- The radioactive Earth. The prospectors ; How old is the earth? ; A new property of matter? -- Speculations. Early theories ; Radioactivity and probability ; Kinetic models of the atom -- Radioactivity and chemistry. The rise of radiochemistry ; Radioactive genealogy ; Chemistry of the imponderable ; Inseparable radioelements ; Isotopes ; Displacement laws ; The end of the lines ; More isotopes -- Inside the atom. Building blocks ; Bombarding atoms ; The nuclear atom ; The nucleus and the periodic table ; The gamma rays ; Theories of the nucleus -- Sequel. War! ; Radioactivity during World War I ; From radioactivity to nuclear and particle physics -- Measuring and using radioactivity. Methods and instruments. Crucial choices ; Standardizing the measures ; Innovations ; Size, money, and machines -- Radioactivity, medicine, and life. Unpleasant surprises ; From burns to treatments ; Rays and other organisms ; Miracle cure? ; Radioactive spas ; Dangers in the laboratory -- New industries. Early industry ; Soaring demand and new institutions ; Paint that glowed in the dark ; A new poison ; Fission, bombs, and the uranium rush ; Radioactivity and the oil industry -- Beyond the story. Radioactivity's prime movers. Technology, resources, and professional changes ; Individuals ; Research groups ; Scientific ideals and culture ; Mentors and models ; Age, attitudes, and ambition ; Nationalism -- Radioactivity and timeless questions : the quest for understanding. Models and theories for radioactivity ; Patterns in radioactivity's development ; Radioactivity and ideas about change ; Radioactivity and ideas about matter and energy ; Radioactivity and ideas about continuity and discontinuity ; Eternal conundrums -- The imaginative appeal of a discovery. Mythological and romantic dimensions of radioactivity ; An ongoing task.