  The current methods for realization of the second with the highest accuracy
through the frequency of the hyperfine transition between two levels of the
ground state of the cesium 133 atom are considered. It is discussed possible
ways for the future redefinition of the SI time unit with the help of optical
frequency standards, as well as thorium and super-high frequency standards.
Several constants which can be also used for definitions of the time, mass and
electric current units are analyzed.
