An introduction to the physics of protons and neutrons in the atomic nucleus, with a focus on explaining the different modes of radioactivity, alpha, beta, and gamma decay, as well as different ways of measuring radiation and some of its biological effects. I also discuss some applications of radioactivity in radiometric dating, radiotherapy, nuclear power, and nuclear weapons. I conclude by discussing a few popular misconceptions about radioactivity. Recommending prelistening is Episode 8: History of the Atom.