We describe the principles and applications of two essential methods for measurement of electron temperature and the density of non-equilibrium plasmas including atmospheric-pressure discharge, based on optical emission spectroscopic (OES) measurement, i.e., line intensity measurement assisted with a collisional radiative model, and continuum spectrum measurement as electron-neutral bremsstrahlung. For the former scheme, we introduce three practical procedures to interpret the OES results based on the excitation kinetic model, which are the “selection of the dominant elementary processes”, the “measurement of two pairs of line intensities”, and the “conversion of the excitation temperature to the electron temperature”. In the last part, examples are shown for the demonstration of the continuum measurement, which has the potential to deduce the electron energy distribution function, followed by its future prospects.