  The symmetry energy contribution to the nuclear Equation of State (EoS)
impacts various phenomena in nuclear astrophysics, nuclear structure, and
nuclear reactions. Its determination is a key objective of contemporary nuclear
physics with consequences for the understanding of dense matter within neutron
stars. We examine the results of laboratory experiments that have provided
initial constraints on the nuclear symmetry energy and its density dependence
at and somewhat below normal nuclear matter density. Some of these constraints
have been derived from properties of nuclei. Others have been derived from the
nuclear response to electroweak and hadronic probes. We also examine the most
frequently used theoretical models that predict the symmetry energy and its
slope. By comparing existing constraints on the symmetry pressure to theories,
we demonstrate how the contribution of the three-body force, an essential
ingredient in neutron matter models, can be determined.
