  1. Introduction
  2. Possible violation of the Gottfried sum rule
  2.1 Gottfried sum rule 2.2 Early experimental results 2.3 NMC finding and
recent progress 2.4 Small $x$ contribution 2.5 Nuclear correction: shadowing in
the deuteron 2.6 Parametrization of antiquark distributions
  3 Expectations in perturbative QCD
  3.1 Operator product expansion 3.2 Perturbative correction to the Gottfried
sum
  4 Theoretical ideas for the sum-rule violation
  4.1 Lattice QCD 4.2 Pauli exclusion principle 4.3 Mesonic models 4.3.1
Meson-cloud contribution 4.3.2 Chiral models 4.3.3 Anomalous $Q^2$ evolution
4.4 Diquark model 4.5 Isospin symmetry violation 4.6 Flavor asymmetry ubar-dbar
in nuclei 4.7 Relation to nucleon spin 4.8 Comment on effects of quark mass and
transverse motion
  5 Finding the flavor asymmetry ubar-dbar in various processes
  5.1 Drell-Yan process 5.1.1 Fermilab-E866 results 5.2 W and Z production 5.3
Quarkonium production at large $x_{_F}$ 5.4 Charged hadron production 5.5
Neutrino scattering 5.6 Experiments to find isospin symmetry violation
  6 Related topics on antiquark distributions
  7 Summary and outlook
