The goal of the present work is to study high pressure non-equilibrium plasma discharges in chemically reactive systems. In this work, we present coupled computational studies of high pressure nanosecond pulsed plasmas for multiphysics applications ranging from plasma assisted combustion ignition, large gap thermal breakdown, to electric discharge in liquids for fuel reforming and biomedical applications. In the first part of the work, we report the results of a computational study which explores argon surface streamers as a low-voltage mechanism for thermal breakdown of large interelectrode gaps and investigate the effect of impurities (molecular oxygen) on the development of continuous surface streamer channels under atmospheric-pressure conditions. In pure argon, a continuous conductive streamer successfully bridges the gap between two electrodes indicating high probability of transition to arc. Presence of oxygen impurities in small concentrations (less than 5%) is found to be conducive to streamer induced thermal breakdown as it reduces the threshold voltage of streamer formation and minimizes unwanted streamer branching effects while maintaining a high probability of streamer to arc transition. Higher oxygen impurity levels > 5% are found to significantly deteriorate the continuous conductivity of streamer channel and lead to a much lower probability for transition to thermal arcs. In the second part of the work, we present a computational study of nanosecond streamer discharges in helium gas (He) bubbles suspended in distilled water (H₂O) for liquid reforming applications. The model takes into account the presence of water vapor in the gas bubble for an accurate description of the discharge kinetics. The objective is to study the kinetics and dynamics of streamer evolution and maximize active species production within the gas bubbles which is the quantity of interest for plasma processing of liquids. We investigate two parameters, namely a) trigger voltage polarity and b) the presence of multiple bubbles, which are found to significantly influence the characteristics of the discharge in gas bubbles. A substantial difference is observed in initiation, transition and evolution stages of streamer discharge for positive and negative trigger voltages. The volumetric distribution of species in the streamer channel is more uniform for negative trigger voltages on account of the formation of multiple streamers. In case of the presence of more than one gas bubble, we see the phenomenon of streamer hopping between bubbles where the high electric field in the sheath of the first bubble triggers the streamer discharge in the adjacent bubble. The presence of multiple immersed bubbles reduces the breakdown voltage of the plasma discharge and results in more uniform generation of active species. It is concluded that a negative pin trigger with multiple immersed gas bubbles maximizes the active species generation which is conducive to plasma assisted liquid reforming applications. In the final part of the work, a coupled two-dimensional computational model of nanosecond pulsed plasma induced flame ignition and combustion for a lean H₂ – air mixture in a high pressure environment is described. The model provides a full fidelity description of plasma formation, combustion ignition, and flame development. We study the effect of three important plasma properties that influence combustion ignition and flame propagation, namely a) plasma gas temperature, b) plasma-produced primary combustion radicals O, OH, and H densities, and c) plasma-generated charged and electronically excited radical densities. Preliminary zero-dimensional studies indicate that plasma generated trace quantities of O, OH and H radicals drastically reduces the ignition delay of the H₂ – air mixture and becomes especially important for high pressure lean conditions. Multi-dimensional simulations are performed for a lean H₂ – air mixture (φ=0.3) at 1 and 3.3 atm and a range of initial tem- perature from 1000 - 5000 K. The plasma is accompanied by fast gas heating due to N₂ metastable quenching that results in uniform volumetric heating in the interelectrode gap. The spatial extent of the high temperature region generated by the plasma is a key parameter in influencing ignition; a larger high temperature region being more effective at initiating combustion ignition. Plasma generation of even trace quantities (∼ 0.1%) of primary combustion radicals, along with plasma gas heating, results in a further fifteen-fold reduction in the ignition delay. The radical densities alone did not ignite the H₂ – air mixture. The generation of other plasma specific species results only in a slight ∼ 10 % improvement in the ignition delay characteristics over the effect of primary combustion radicals, with the slow decaying ions (H₂⁺, O₂⁻, O⁻ ) and oxygen metastable species (O₂ [superscript a1], O₂ [superscript b1], O₂ [superscript *]) primarily contributing to com- bustion enhancement. These species influence the ignition delay, directly by power deposition due to quenching, attachment and recombination reactions, and indirectly by enhancing production of primary combustion radicals.