This lecture reviews the concepts of optical atomic clocks and their advantages in comparison to established atomic clocks in the microwave range. Several suitable transition frequencies in laser-cooled and trapped atoms and ions can now be realized with a relative uncertainty in the low 10^–18 range. Laser oscillators can be stabilized to the atomic transitions and are used to measure and to distribute the signals. This high accuracy and the opportunity to compare different atomic reference transitions enable experimental tests of fundamental principles and searches for “new physics” like violations of the Einstein equivalence principle. Some options and routes towards a redefinition of the SI second in terms of an optical frequency are discussed.