Flavor symmetries of parton distributions

The recent progress on our understanding of the flavor structure of the 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 degree of freedom in the description of the nucleon sea. Implications of the meson clouds on other aspects of nucleon parton distributions and possible future measurements for testing various theoretical models are also discussed.