  Ultracold neutrons provide a unique tool for the study of neutron properties.
An overview is given of the ultracold neutron (UCN) source at PSI, which
produces the highest UCN intensities to fundamental physics experiments by
exploiting the high intensity proton beam in combination with the high UCN
yield in solid deuterium at a temperature of 5K. We briefly list important
fundamental physics results based on measurements with neutrons at PSI.
