Flavor asymmetry of antiquark distributions in the nucleon

Violation of the Gottfried sum rule was suggested by the New Muon Collaboration in measuring proton and deuteron F2 structure functions. The finding triggered many theoretical studies on physics mechanisms for explaining the antiquark flavor asymmetry ¯ − ¯ d in the nucleon. Various experimental results and proposed theoretical ideas are summarized. Possibility of finding the flavor asymmetry in Drell-Yan experiments is discussed together with other processes, which are sensitive to the ¯ ¯ d asymmetry.