The application of novel plasma devices has shown promise for the enhancement of ignition and flame-holding in combustion systems. However, the mechanism by which different discharge types interact with the combustion process is still largely unknown. This work explores the plasma/combustion interaction of nanosecond duration discharges utilizing both high density localized discharges and low density diffuse discharges in subsonic and supersonic flows. A variety of optical techniques, including tunable diode laser absorption spectroscopy (TDLAS), kilohertz rate OH planar laser induced fluorescence (PLIF), optical emission spectroscopy (OES), high speed schlieren imaging, and chemiluminescence imaging are used to explore the plasma and combustion processes in situ. In high density discharges, it is found that repetitive application of the nanosecond discharges leads to improved ignition probability and flame-development time at high pulse frequencies (>10 kHz), indicating some pulse-to-pulse coupling important for the ignition process. To explore the kinetics involved in this process, molecular concentration measurements are utilized to track species evolution in the plasma, and a modeling architecture and kinetic model are developed to shed light on the reaction scheme. It is found that high energy electrons initiate chain propagating or branching reactions which can maintain oxidation at temperatures far below the autoignition threshold. In high frequency pulsing, these kinetic effects result in a positive feedback loop in which radical species are produced by both the plasma and combustion reaction processes in series, leading to the observed ignition enhancement.