This book discusses things which are, at first sight, absolutely unrelated. What can elementary particle interactions and tunable dye-lasers, T-invariance and Doppler broadening of spectral lines, the hyperfine structure of bismuth and parity nonconservation have in common? In recent years, however, a new direction in physics research has emerged, within the framework of which these notions proved to be closely related. We mean the study of weak interactions of elementary particles by methods of atomic spectroscopy.