  Neutron stars are some of the densest manifestations of massive objects in
the universe. They are ideal astrophysical laboratories for testing theories of
dense matter physics and provide connections among nuclear physics, particle
physics and astrophysics. Neutron stars may exhibit conditions and phenomena
not observed elsewhere, such as hyperon-dominated matter, deconfined quark
matter, superfluidity and superconductivity with critical temperatures near
${10^{10}}$ kelvin, opaqueness to neutrinos, and magnetic fields in excess of
$10^{13}$ Gauss. Here, we describe the formation, structure, internal
composition and evolution of neutron stars. Observations that include studies
of binary pulsars, thermal emission from isolated neutron stars, glitches from
pulsars and quasi-periodic oscillations from accreting neutron stars provide
information about neutron star masses, radii, temperatures, ages and internal
compositions.
