CR-39 plastic nuclear track detectors have proved to be one of the most useful nuclear track detectors in physics research. They have made significant contributions to research in the fields of particle radiation at aviation altitudes, in space, and in the natural environment. Other uses have included radiation risk estimation, the search for magnetic monopoles and the investigation of energetic particles generated by the low energy nuclear reactions (LENRs) in condensed matter nuclear science (CMNS). This book describes the methods and applications using CR-39 detectors in several important physics research areas and presents results obtained.