Plasma chemistry is most important subject for plasma-processing systems and applications. Basically, it is controlled by highly active species or by the higher temperature properties of the plasmas. In this chapter, clarity about plasma as physics or chemistry has been clearly brought out. Also, chemistry of nonthermal plasma applications specially for dielectric barrier discharge-based VUV/UV excimer light sources has been discussed. In the chemically active plasma discharges, in situ diagnostics are not possible due to contamination issues. The passive optical emission spectroscopy diagnostic is a better solution. The recent trends of plasma spectroscopy diagnostics in concurrence with the plasma chemistry understanding has been presented.