He, Calvin ZThe viability of using relativistic Thomson scattering (RTS) as an intensity gauge of intense(I ≳ 2 × 1018 W/cm2) lasers is explored. Theory motivating measurement of the RTS second harmonic spectrum as well as angular distribution as possible diagnostics of the laser intensity are developed and presented. Experiments conducted at the Centro de Láseres Pulsados (CLPU) laser facility in Salamanca, Spain measuring the second harmonic spectrum radiated along E as well as the angular distribution along the E-k plane are described and their results summarized. We conclude that measurements of the second harmonic spectrum provide a means for determining the laser peak intensity via the corresponding Doppler shifted onset wavelength, provided that the intensity is in the range of ∼ 10^18 to ∼ 10^19 W/cm^2. Measurement of the angular distribution along the E-k plane, on the other hand, does not provide features that are useful for indicating laser intensity. Possible paths forward in the study of RTS are discussed.enDissertationPhysics