SummaryA discussion of geological description and an account of the Descriptive System LSDØ2 are given. It is shown that by examination of the aims of description and of its main instrument, the geological observer, a set of principles and objectives may be established in order to design descriptive systems. The application of these principles is demonstrated by an account of the theory and practical aspects of the Lithostratigraphic Descriptive System LSDØ2.