Depending on various plasma conditions, such as size, composition, densities and temperatures, the electron density and related quantities can be measured using a number of techniques. Of these, Langmuir probes (Tonks and Langmuir 1929, Hutchinson 1987) provided the first means to infer local values of the electron density, mostly at relatively low densities. Much more recently, Thomson scattering of laser light has become a method of choice for localized electron density measurements (Kunze 1968, Evans and Katzenstein 1969, DeSilva and Goldenbaum 1970, Sheffield 1975) over a range of about 1011 to 1021 cm−3. Then there are the usually inherently most accurate interferometric techniques, mostly laser-based as well (Jahoda and Sawyer 1971, Hauer and Baldis 1988), which span a similar range, recently extended to Ne ≈ 3 × 1021 cm−3 (DaSilva et al. 1995), but provide localized values of the electron density only indirectly. Using two or more wavelengths, one can, however, separate the free-electron contribution (1.37) to the refractive index from any boundstate contribution (2.103). In single-species, partially ionized plasmas, this boundstate contribution is a direct measure of the neutral atom density. Interferometric local values of the density can, in principle, be determined by methods analogous to those discussed for emission or absorption measurements in section 8.5.