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Abstract. Noctilucent clouds appear during the summertime at high lantudes near the top of the mesosphere. In this review, the observattonal facts about them, obtained from ground level, by rocket sounding and from orbiting spacecraft, are reviewed. The data are not sufficiently clear and unambiguous to permit dogmatic assemon about the origin and nature of the clouds. They seem to be ice particles nucleated at very low pressures and temperatures by either meteoric smoke or by atmospheric ions. Wavepatterns m the clouds may well result from quite close relations between the troposphere and the mesosphere The very existence of the clouds leads to difficulties in explaining why there is so much water vapour at this great height in the atmosphere. To try to predict the microscopic behaviour of the cloud particles leads one into assessment of the relative importance of radiometer effects, radiauon balance, Brownian movement, electric polanzanon and the influence of Coulomb attraction on the growth of large clustered ions. Finally, a hst ~s gtven of published sources of observanonal data.

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