Direct and wavelength modulated methods of tunable diode laser absorption spectroscopy (TDLAS) techniques were applied to hypersonic and rarefied gas environments. Three experimental campaigns are discussed including direct absorption spectroscopy of a steam ejector flow, wavelength modulated TDLAS of scramjet combustion in an expansion tunnel, and temperature measurement of atomic oxygen in a plasma generator. The literature relevant to TDLAS is reviewed and the theoretical bases for the two techniques are presented. Each campaign required innovative solutions to experimental problems but, in the end, TDLAS was successfully applied on a proof-of-concept basis in each environment. The pressure and temperature in a 1 kms −1 steam ejector flow were measured using direct TDLAS.Three methods of analysing the data were compared for accuracy and ease of use. The best method was found to be using the Levenburg-Marquardt method to perform a non-linear least-squares minimisation of the residual between a Voigt fit of simulated and measured data. Absolute average residual errors of less than 0.1% were achieved together with pressure relative error of ±2% and temperature errors of ±10 K. Wavelength modulated TDLAS was investigated as a means to measure parameters in the combustor and nozzle of a scramjet in an expansion tunnel. The scramjet operated at an inlet Mach number of approximately 10 and a static pressure of 120 Pa simulating flight at approximately 30 km altitude. The test time was of the order of 1 ms. Hydrogen fuel was injected via five holes in the scramjet inlet ramp. Two laser diodes operating near 1392 nm were used to probe the scramjet nozzle flow. The lasers were simultaneously sinusoidally modulated in power and wavelength at a frequency near 1.0 MHz. Tests were conducted at various fuel injection pressures into both air and nitrogen test gas.Combustion was detected in the scramjet nozzle. Results are presented together with recommendations for future research. The atomic oxygen concentration in a microwave generated plasma generated by an Evenson cavity was measured using TDLAS to probe the oxygen triplet at 844.6 nm. The oxygen atoms in the plasma were highly excited and emitted radiation that could have overwhelmed the laser absorption. Any spectral features were expected to be narrow as the plasma pressure was between 80 Pa and 300 Pa. The measured intensity ratios were used to calculate the number density of oxygen atoms in the upper states with a relative error of approximately 5%. The calculated number density, mole fraction of atomic oxygen and intensity ratio were highly sensitive to the temperature as was expected given the exponential form of Boltzmann's equation. A brief analysis was also presented of a TDLAS system that could detect atomic oxygen in reflected shock or expansion tunnel experiments. Whilst the system was not implemented for this study, the information presented may assist future researchers to develop an atomic oxygen TDLAS system for free-piston hypersonic tunnels. Th...