  White dwarfs and neutron stars are stellar objects with masses comparable to
that of our sun. However, as the endpoint stages of stellar evolution, these
objects do not sustain any thermonuclear burning and therefore can no longer
support the gravitational load of their own mass by generating thermal
pressure. Rather, matter in their interiors is compressed to much higher
densities than commonly found in normal stars, and pressure is created by
degenerate fermion kinetic energy and particle interactions. As a result, white
dwarfs and neutron stars offer unique cosmic laboratories for studying matter
at very high densities. In this review we discuss the basic properties of
condensed matter at extreme densities and summarize the extent to which these
properties can be examined by observations of compact objects.
