  A surprisingly large asymmetry between the up and down sea quark
distributions in the nucleon has been observed in recent deep inelastic
scattering and Drell-Yan experiments. This result strongly suggests that the
mesonic degrees of freedom play an important role in the description of the
parton distributions of the hadronic sea. In this article, we review the
current status of our knowledge of the flavor structure of the nucleon sea. The
implications of various theoretical models as well as possible future
measurements are also discussed.
