Gamma rays and neutrons are used to measure the elemental compositions of planetary surfaces, which provide fundamental information to our understanding of how planetary bodies form and have changed over time. In this talk, I describe the technique of planetary gamma-ray/neutron spectroscopy, including what it is and how it is carried out. I will then tour the solar system, stopping off at locations where compositional measurements have made (or will make) important contributions to planetary science. Finally, gamma-ray and neutron detectors traveling around the solar system provide opportunities for scientific exploration beyond planetary science. Towards that end, I briefly review how planetary gamma-ray/neutron experiments contribute to areas of space and solar physics, astrophysics, and fundamental physics.
