Top Quark Physics

Top quark studies at future $e^+e^-$ colliders are considered. Two issues are discussed: {a --}Some results are presented on the decays of top quarks. Energy distributions of charged leptons and neutrinos in $t\to bW\to be^+\nu$ and jets in $t\to bW\to b\bar du$ decays are sensitive to the structure of $tbW$ vertex. Distributions of charged leptons from top decays are particularly useful in polarization studies whereas neutrinos are sensitive to deviations from the Standard Model. {b --}Recent calculations are reviewed on the top quark pair production in $e^+e^-$ annihilation. The differential cross sections in the threshold region can lead to an accurate determination of the top quark mass and the interquark potential. The effects of the top-Higgs Yukawa coupling and some higher order QCD corrections are also under control.