The discovery of the valence and sea quark structures in the nucleons inspired the formulation of Quantum Chromodynamics (QCD) as the gauge eld theory governing the strong interaction. A surprisingly large asymmetry between the up and down sea quark distributions in the nucleon was observed in semi-inclusive DIS and the Drell-Yan experiments. In this talk, I discuss the current status of our knowledge on the avor structure of the nucleon sea. I will also discuss the progress in identifying the "intrinsic" sea components in the nucleons.