[en] Optical aerosol measurement techniques are used as light scattering devices in nuclear aerosol research. The underlying scattering theory, the Mie theory, is known since 1908. Nevertheless, a practical application of light scattering instruments has only recently become possible. The use of light scattering techniques for particle measurement is many-fold, scattering, absorption and extinction methods can be applied to single particles or particle ensembles. The Mie theory gives excellent agreement with measured data as long as the underlying assertions are met. The assumptions are that particles are spherical and that their complex refractive index is known. With nuclear aerosols this is true only for liquid sodium or water droplets, in which cases the use of in situ measuring optical particle spectrometers is highly recommended. For non-spherical particles the Mie equations hold only approximately, the degree of the approximation varies with particle size. In these cases optical spectrometers can be recommended after careful calibration. In all cases. however, the advantage of optical techniques is that in situ measurements can be done without influencing the aerosol and with high time resolution. These properties are extremely useful for measuring dynamic aerosol systems, or particles which cannot be sampled without changing their properties. Some commercial and a variety of special laboratory prototype instruments exist capable of measuring particles sizes > 0.1 micrometer in the concentration range up to 10⁵ P/cm³. Photometer type instruments have larger ranges but the interpretation of the meter reading is difficult in most cases