The existence of solid particles in the atmospheric region of 80-90 km has been known since the 1880-ies. These particles are formed summertime at high latitudes in the Northern and Southern hemisphere as the ambient temperature decreases to below 130 K and can be observed as noctilucent clouds. Most of the available information on the physical properties of the aerosol particles, such as the effective particle radius of less than 100 nm have been derived from optical studies, notably ground-based and rocketborne polarimetric measurements, satellite-based multiple-wavelength analysis of the ligth scattered by the clouds as well as the faint but detectable long-path extinction in the visible and IR spectrum. Through satellite measurements of the IR extinction it is now confirmed that NLC particles indeed consist of solid H2O and in addition, recent far-IR spectra taken by from satellite spectrometers have recorded the thermal emission predicted for low-temperature ice from NLC particles.

This contribution is an attempt to summarize the merits and shortcomings of the various techniques applied so far, in particular concerning the so far unknown habit and shape of the NLC grains and their dependence on the microphysical processes of particle growth and decay under mesospheric conditions.