The flows of materials through the economy are presented in un input-output table. The table includes the resources (both free and "economic") extracted from the natural bodies (air, waters, soil, and "stocks" of minerals, stones, and fossil fuels). For the principle of mass conservation most of the materials used by the economic activity branches are returned to the natural bodies as wastes. A part of the materials flowing through the economic branches are enbodied in stocks (buildings, roads, machinery, goods having a life longer than the year at which the input-output analysis is referred). Keeping into consideration also the material imports and exports, it is possible to evaluate a Gross National Material Product (GNMP), in mass units, formally similar at, and superposable to, the monetary GNP. For Italy in 1995 the GNMP has been evaluated in little more than 600 million metric tons, corresponding at about 11 tons per person per year. The applications of the analysis and the open problems are reviewed.