  Ultracold neutrons (UCN) are free neutrons that can be stored in experimental
setups for several minutes. Some of the most important properties of the
neutron, such as its tiny permanent electric dipole moment and its beta decay
lifetime, are best measured with UCN. Also searches for well motivated but yet
unknown, hypothetical additional interactions are being pursued with UCN. Such
measurements in the field of low-energy, precision physics may have far
reaching implications from particle physics to cosmology. Most experiments are
statistics limited and need high-intensity UCN sources. The UCN source at PSI
is at the forefront of the field and home to the international nEDM
collaboration and its world-leading search for the neutron electric dipole
moment. This article aims at giving an overview of the fascinating research
using ultracold neutrons emphasizing on activities at PSI including various
physics side-analyses which were pioneered by the nEDM collaboration.
