Optical Cavities for Optical Atomic Clocks Atom Interferometry and Gravitational Wave Detection Atomic clocks cold atoms and gravitational waves Part 1: Cavities for Optical Atomic Clocks Thermal noise limited room temperature ULE cavity Isolation from external perturbations Measuring resonator stability Part 2: Cavities for Atom Interferometry Cavity atom optics Fundamental limitations of cavity assisted atom interferometry Gravitational wave detection with cavity assisted atom interferometry 4 mirror large waist cavity with tuneable stability for enhanced atom interferometry Part 3: Cavities for Gravitational wave Detection Near unstable cavities for future gravitational wave detectors Modelling parametric instabilities at Advanced LIGO and ET Summary and conclusions Appendix Devised at the beginning of the 20th century by french physicists Charles Fabry and Alfred Perot the Fabry Perot optical cavity is perhaps the most deceptively simple setup in optics and today a key resource in many areas of science and technology. This thesis delves deeply into the applications of optical cavities in a variety of contexts: from LIGO’s 4 km long interferometer arms that are allowing us to observe the universe in a new way by measuring gravitational waves to the atomic clocks used to realise time with unprecedented accuracy which will soon lead to a redefinition of the second and the matterwave interferometers that are enabling us to test and measure gravity in a new scale. The work presented accounts for the elegance and versatility of this setup which today underpins much of the progress in the frontier of atomic and gravitational experimental physics