We review the physics involved in the production and decay of top quarks in
e^+e^- → ttbar near threshold, with special emphasis on the recent theoretical
study on the decay process of top quarks in the threshold region. The
energy-angular distribution of l^+ in semileptonic top decays is calculated
including the full O(alpha_s) corrections. Various effects of the final-state
interactions are elucidated. A new observable is defined near threshold, which
depends only on the decay of free polarized top quarks, and thus it can be
calculated without bound-state effects or the final-state interactions.
