A brief review is given of the present status of precipitation phenomena in semiconductors. Recent theoretical advances in the analysis of diffusion‐limited precipitation are described. The precipitation of Li and Si is discussed as an example of diffusion‐limited precipitation in which metastable centers are observed, and the precipitation of Cu as an example to which formal nucleation theory appears to apply. A recent theory of the clustering of oxygen in Si is described and its significance in terms of the nucleation process is indicated. It is emphasized that a number of techniques are now available for the observation of tiny aggregates of impurities in semiconductors, and the need for a consistently microscopic theory of nucleation is pointed out.