  Sea quark distributions in the nucleon have naively been expected to be
generated perturbatively by gluon splitting. In this case, there is no reason
for the light quark and anti-quark sea distributions to be different. No
asymmetries in the strange or heavy quark sea distributions are predicted in
the improved parton model. However, recent experiments have called these naive
expectations into question. A violation of the Gottfried sum rule has been
measured in several experiments, suggesting that $\bar u < \bar d$ in the
proton. Additionally, other measurements, while not definitive, show that there
may be an asymmetry in the strange and anti-strange quark sea distributions.
These effects may require nonperturbative explanations. In this review we first
discuss the perturbative aspects of the sea quark distributions. We then
describe the experiments that could point to nonperturbative contributions to
the nucleon sea. Current phenomenological models that could explain some of
these effects are reviewed.
