The theoretical basis, the content and the possible applications of the program 'Monitoring of ecosystems' are considered. A functional definition of ecosystem is proposed, and some features of ecosystems are defined on this basis. The program 'Monitoring of ecosystems' is visualized as a large-scale program of simple measurements of the rates of the main ecosystem processes. The expected results can be utilized in two ways: (1) a comparative knowledge of ecosystem functioning provides the fundamentals of geography of ecosystems, and (2) the constant, repeatable evaluation of a given ecosystem function provides a basis for its rational management.