Plasma processing represents a powerful approach to modification of a range of substrates utilizing an array of chemistries and morphologies. Unfortunately, plasmas are often given a bad rap with respect to the level of reproducibility achievable as well as the uniformity of any given treatment. Nevertheless, new applications for plasmas continue to be developed and they are employed in a vast array of industries to produce high impact, high value products. One strategy for increasing the robustness of plasma surface modification processes lies in increasing our understanding of the fundamental chemistry of the gas phase chemistry in plasmas, the resulting film chemistry and perhaps most importantly, the gas-surface interface. This talk will focus on recent work in our laboratory that explores not only the impact of the plasma on the surface, but also the effect of the substrate on the plasma chemistry. Data on systems used for plasma assisted catalysis (PAC) and plasma modification of nanostructured sensor materials will be presented. As one example, we have combined a range of spectroscopy techniques, materials characterization tools, and plasma-surface interface studies to reveal that the presence of a catalytic substrate in the plasma system results in significant changes in the plasma chemistry, most notably affecting the internal temperatures (vibrational, rotational) of various plasma species. Changes in plasma composition as well as substrate surface chemistry and morphology were also observed. Connections between these results and other trends we observe at the plasma-surface interface will be discussed.