In this paper the authors give a survey of biological phenomena (both at the cellular level and in organisms), for which the term 'differentiation', used in the widest sense, has a meaningful application. The last chapter deals with some general aspects of differentiation and connected phenomena, such as polarity, symmetry, loss of potencies, dedifferentiation, redifferentiation, adifferentiation, regulation and integration. To prevent this article becoming too voluminous, the authors had to refrain from extending the treatment of differentiation to non-living matter and to human society.