In the late 1950s, scientists at General Electric — among them the author — discovered that when mica is exposed to energetic charged particles, the particles leave latent tracks in the material. It soon turned out that glass, plastics, or certain other materials can be similarly treated. This discovery paved the way not only for a new and useful method of measuring radioactivity, it has also found widespread applications in other fields, ranging from geology and materials science to archaeology and art history. Fleischer presents the history of these developments and discusses the applications of the technique in a way that will interest anyone even with only a minimal knowledge of physics.