Neutrons are a powerful tool for particle and nuclear physics and they are ideal probes for quantum investigations and gravitational physics. The related experiments depend on the availability of high densities and fluxes for cold and ultracold neutrons. Contrary to the usual particle physics experiments, which take place at the highest possible energies of particles, these experiments with neutrons have energies, which are even much lower than those of ordinary gas molecules. Rather recently, new proposals for ultracold neutron sources using advanced moderation and pumping processes make density gains in the order of 10 3 – 10 4 feasible. Also, b-decay experiments with cold neutrons can make use of the pulse structure of ESS for background suppression achieving a decay rate, which is 3 orders of magnitude higher than present experiments.