Problems of the origin and physical characteristics of ice crystal and cloud drop nuclei continue to receive considerable attention. There is good evidence that sea salt particles play only a minor role in the world supply of cloud drop nuclei, although they do provide the largest nuclei that are of importance in the initiation of coalescence rain. The mode of action of ice nuclei is still inadequately understood; there is increasing disillusionment with the concept that any direct correspondence of ice nuclei measurements and ice crystals in clouds should exist. Possible mechanisms for ice crystal production other than by solid particle nucleation have been investigated. Increasing interest has developed on the role of pollution products in both drop and crystal nucleation.