  A review is made of constraints on the nuclear symmetry energy parameters
arising from nuclear binding energy measurements, theoretical chiral effective
field predictions of neutron matter properties, the unitary gas conjecture, and
measurements of neutron skin thicknesses and dipole polarizabilities. While
most studies have been confined to the parameters $S_V$ and $L$, the important
roles played by, and constraints on $K_{\rm sym}$, or, equivalently, the
neutron matter incompressibility $K_N$, are discussed. Strong correlations
among $S_V, L$, and $K_{N}$ are found from both nuclear binding energies and
neutron matter theory. However, these correlations somewhat differ in the two
cases, and those from neutron matter theory have smaller uncertainties. To 68\%
confidence, it is found from neutron matter theory that $S_V=32.0\pm1.1$ MeV,
$L=51.9\pm7.9$ MeV and $K_N=152.2\pm38.1$ MeV. Theoretical predictions for
neutron skin thickness and dipole polarizability measurements of the
neutron-rich nuclei $^{48}$Ca, $^{120}$Sn, and $^{208}$Pb are compared to
recent experimental measurements, most notably the CREX and PREX neutron skin
experiments from Jefferson Laboratory. By themselves, PREX I+II measurements of
$^{208}$Pb and CREX measurement of $^{48}$Ca suggest $L=121\pm47$ MeV and
$L=-5\pm40$ MeV, respectively, to 68\% confidence. However, we show that
nuclear interactions optimally satisfying both measurements imply $L=53\pm13$
MeV, nearly the range suggested by either nuclear mass measurements or neutron
matter theory, and is also consistent with nuclear dipole polarizability
measurements. This small parameter range implies $R_{1.4} = 11.6\pm1.0$ km and
$\Lambda_{1.4} = 228^{+148}_{-90}$, which are consistent with NICER X-ray and
LIGO/Virgo gravitational wave observations of neutron stars.
