  I review attempts made to determine the properties of neutron stars. The
focus is on the maximum mass that a neutron star can have, or, conversely, the
minimum mass required for the formation of a black hole. There appears to be
only one neutron star for which there is strong evidence that its mass is above
the canonical 1.4 solar masses, viz., Vela X-1, for which a mass close to 1.9
solar masses is found. Prospects for progress appear brightest for studies of
systems in which the neutron star should have accreted substantial amounts of
matter.
