Slow neutrons are used in a large number of experiments to study the physics of particles and their fundamental interactions as well as their underlying symmetries. Some of these experiments search for manifestations of "new physics" in order to verify or exclude theories leading beyond the present Standard Model of particle physics. Other experiments provide high precision data for standard-model parameters on the first generation of elementary particles. They also serve as input for work in other fields like cosmology, astrophysics or neutrino physics. Finally, there is a large number of experiments on the foundations of quantum mechanics, mostly in the field of neutron optics.