Precipitation of inorganic compounds or metals from aqueous solutions can either be physical (crystallization) or chemical in nature. Chemical processes are varied: they may be due to hydrolytic action, due to to ionic interaction, or due to reduction, i.e., transfer of electrons. Transfer of electrons in reduction processes may take place homogeneously or heterogeneously. In the first case it may be ionic or non-ionic in nature. Heterogeneous processes may be electrochemical of electrolytic. Examples of interest to hydrometallurgy are given. Translated into Turkish by Prof. Murat Erdemoglu, Inönü Üniversitesi, Malatya, from Proceedings of XVIII International Mineral Processing Congress, Sydney, Australia 1993, pp. 1323–1328