We examine functional geography as a part of the theory of functional systems, the subject matter of which now involves the cause-effect relationships of the influence of the factors and conditions on processes and phenomena of a different origin. Within the framework of this scientific and geographical direction, most attention is concentrated on the study into the environmental conditions, the methods of taking them into account in calculation formulas, analysis of critical levels of variability, and on the search for environmental invariants. By taking into consideration the distinguishing features of the environment, geography provides an informational basis for solving applied problems in other fields of knowledge, proves the environmental relativity and geographical determinism of the manifestation of the laws of the functioning of nature and society. The article discusses the relationship between structure, dynamics and functioning in geographical studies, presents the theoretical and mathematical bases for modelling the functional features of geosystems and substantiates the functional approach and the corresponding models. The functions of geosystems are investigated and modelled in the form of the interaction of their components, the characteristics of which are regarded as the factors that jointly influence the degree of manifestation of these functions. It is shown that the possibility of explaining the regularities of the functioning of geographical systems is ensured are provided by the use of the logarithmic scale of the characteristics of the influencing factors.Keywords: functional systems, geosystem functions, environmental factors and conditions, mathematical models of cause-effect relationships, logarithmic scales.