The theory of electromagnetic wave propagation in a cold plasma is onl
y briefly treated. Optical approximations made for diagnostic applicat
ions are mentioned. Refractive index measurements via FIR-interferomet
ry are discussed and an electron density profile evaluation is illustr
ated. The principle of microwave reflectometry is presented and exampl
es from fixed frequency and pulsed radar reflectometry are shown. The
theoretical background of electron cyclotron emission is given in a si
mplified version and examples of electron temperature measurements on
TEXTOR are presented.