The development of diagnostics based on laser-absorption spectroscopy for combustion applications has been an important and active field of research over the past two decades due to the advantages of this non-intrusive optical sensing technique compared to traditional sampling-based sensing methods. Tunable diode laser (TDL) sensors, in particular, have shown the ability to provide in situ, time-resolved, line-of-sight measurements of temperature, gas species concentration, velocity, density, mass flux, and pressure in a variety of combustion environments. This thesis explores three new areas of TDL research: (a) extended near-infrared (NIR) diagnostics, (b) sensing under high-pressures, and (c) applications to chemical kinetics. Water vapor (H2O) and carbon dioxide (CO2) are attractive sensing targets for hydrocarbon-fueled systems as they are primary combustion products and their concentrations can be interpretrated to indicate combustion progress and efficiency. Both these gases have absorption spectra in the infrared (IR) region. Most previous TDL absorption sensors were designed to exploit robust telecommunications diode lasers and optical fiber technology in the 1.3-1.6 [mu]m (NIR) wavelength region. Recent developments in semiconductor diode-laser technology have extended the range of continuous wave (CW) room-temperature single-mode diode lasers to 2.9 [mu]m, allowing access to stronger vibrational bands of H2O and CO2 in the extended-NIR region. The first combustion diagnostics in the extended-NIR wavelength were demonstrated as part of this thesis work. The sensors were designed by selecting optimal transitions and then measuring the pertinent spectroscopic parameters in controlled laboratory environements. These sensors were then tested in the combustion environments of a flat flame and shock tube to validate their performance. These new sensors provide enhanced sensitivity and improved accuracy compared to previous TDL diagnostics. As part of this work, a novel diagnostic based on wavelength modulation spectroscopy (WMS) of CO2 was developed to make precise measurements of temperature behind reflected shock waves. This temperature diagnostic achieved an unprecedented uncertainty of