  The recent progress on our understanding of the flavor structure of
unpolarized and polarized nucleon sea is reviewed. The large flavor asymmetry
between the up and down sea quark distributions is now well established. This
asymmetry strongly suggests the importance of the mesonic degrees of freedom in
the description of the nucleon sea. The strong connection between the flavor
structure and the spin structure of the nucleon sea is emphasized. Possible
future measurements for testing various theoretical models are also discussed.
