Recent studies of phenomena in molecular non-equilibrium microwave plasmas, which have been performed within the framework of the DFG- Sonderforschungsbereich 198, project A12, are presented in this review article. Special attention is devoted to the properties and kinetics of transient molecular species. The experimental methods used were optical emission and absorption spectroscopy. The main objectives of this contribution were threefold: (i) the determination of the degree of dissociation of hydrogen in plasmas, (ii) the quantitative detection of the methyl radical, and (iii) plasma process monitoring using quantum cascade lasers.