The theory of electromagnetic wave propagation in a cold plasma is only briefly treated. Optical approximations made for diagnostic applications arc mentioned. Refractive index measurements via FIR-interferometry are discussed and an electron density profile evaluation is illustrated. The principle of microwave reflectometry is presented and examples from fixed frequency and pulsed radar reflectometry are shown. The theoretical background of electron cyclotron emission is given in a simplified version and examples of electron temperature measurements on TEXTOR are presented.