Plasticity of crystals in a microscopic viewpoint, especially on the basis of dislocation (linear defects in crystal) motion, is detailed from the data obtained mainly by the strain-rate cycling tests associated with ultrasonic oscillation for several kinds of alkali-halide single crystals at low temperatures. Topics discussed include process for thermal activated overcoming of the short-range obstacle by a dislocation; strain-rate cycling tests associated with ultrasonic oscillation; influence of state of impurities on the effective stress due to the impurities; effective stress and critical temperature of KCl doped with various divalent cations; and the Friedel relation and thermally activated flow parameters of KCl doped with various divalent cations.