Various attempts have been made in recent years to overcome the difficulties inherent in the study of clouds by reproducing them on a small scale in the laboratory. The present article briefly reviews some of these experiments, particularly those relating to the formation of ice crystals, and discusses the extent to which the laboratory results accord with what is known of natural clouds. Recent experiments on the artificial stimulation of rainfall are also examined for the light they throw on the part played by ice nuclei in the formation of rain.