  We summarize our current knowledge of neutron star masses and radii. Recent
instrumentation and computational advances have resulted in a rapid increase in
the discovery rate and precise timing of radio pulsars in binaries in the last
few years, leading to a large number of mass measurements. These discoveries
show that the neutron star mass distribution is much wider than previously
thought, with 3 known pulsars now firmly in the 1.9-2.0 Msun mass range. For
radii, large, high quality datasets from X-ray satellites as well as
significant progress in theoretical modeling led to considerable progress in
the measurements, placing them in the 9.9-11.2 km range and shrinking their
uncertainties due to a better understanding of the sources of systematic
errors. The combination of the massive neutron star discoveries, the tighter
radius measurements, and improved laboratory constraints of the properties of
dense matter has already made a substantial impact on our understanding of the
composition and bulk properties of cold nuclear matter at densities higher than
that of the atomic nucleus, a major unsolved problem in modern physics.
