Introduction 641
I. Atomic and Molecular Frequency and Time Standards 642
1. Molecular and Atomic Generators 642
2. Cesium Frequency Standards 647
3. Other Frequency Standards 649
4. Frequency Stabilization 651
II. Use of Highly Stable Frequency and Time Standards to Check General and Special Relativity 653
1. Possible Experiments to Check General Relativity 653
2. First-Order Experiments to Check Special Relativity 666
III. Use of Atomic and Molecular Frequency Standards for the Investigation of Cosmological Effects 669
Conclusion 670
Appendix. Derivation of Formula for Relativistic Red Shift of the Frequency of a Spectral Line 670
References Cited 671