  The flavor asymmetry of the nucleon sea is discussed in an unquenched quark
model for baryons in which the effects of quark-antiquark pairs (u anti-u, d
anti-d and s anti-s) are taken into account in an explicit form. The inclusion
of q anti-q pairs leads automatically to an excess of anti-d over anti-u quarks
in the proton, in agreement with experimental data.
