Interferornetric methods for determining the electron concentration Ne in thermal plasmas are reviewed. Such methods depend on measuring the contribution to the refractive index of the free electrons for a probing em wave of suitable frequency. This contribution depends on the ratio of the wave frequency f to the plasma frequency fp ∞ Ne 1/2 and in practice (f/fp) must be significantly greater than unity. For thermaill plasmis, of interest for various devices and processes, probe frequencies in the range 1011 to 1013 Hz are required, corresponding to wavelengths in the millimeter to mid infrared regions, which are accessible with CO2 laser-pumped gas lasers. The technique may be implemented using a two-path laser interferometer or, in the case of a strongly magnetized plasma, by using Faraday rotation of the plane of polarization. Experiments are detailed in which both these methods were used to measure Ne in seeded combustion plasmas in a mag-netohydrodynamic generator.