“Primary production” refers to energy fixed by plants. The total amount of energy fixed is usually called “gross production.” A certain fraction of gross production is used in respiration by the plants; the remainder appears as new biomass or “net primary production.” Thus for a single plant or a community of green plants: Net Primary Production = Gross Production − Respiration (of Autotrophs) Similar relationships occur in ecosystems except that the organic matter and respiration of heterotrophs must be included. The increase in total organic matter is “net ecosystem production”; respiration is the total respiration of the green plants (autotrophs) and the animal community and decay organisms (heterotrophs). Gross production is of course identical to that of the plant community. Thus for an ecosystem: Net Ecosystem Production = Gross Production − Respiration (of Autotrophs and Heterotrophs) Study of these attributes of terrestrial ecosystems is difficult, both because of the complex interrelations of the processes involved, and because of the problems of working with systems as large as whole forests. Three approaches are in use: (1) Harvest techniques measure weight increase (and caloric equivalent and chemical composition) of net production. A refinement ot this approach based on “dimension analysis” has made possible important recent advances in the study of forests. Other techniques approach gross production and respiration through measurement of exchange of gases, especially CO2. These include: (2) Enclosure studies, involving measurements of CO2 exchange in plastic enclosures of parts of ecosystems and (3) Flux techniques based on measurement of CO2 levels in the environment. All three approaches are being applied to a forest at Brookhaven National Laboratory to determine the production equation of this ecosystem. Results to date have established general ranges of such parameters of ecosystems as total biomass, total surface area of leaves and of stems and branches, rates of decay of organic matter in soils, rates of production of roots, and rates of photosynthesis and respiration under different environmental conditions. In the Brookhaven forest net primary production is 1124 dry g/m2/yr (with an energy equivalent of 492 cal/cm2/yr), and gross production is about 2550 dry g/m2/yr; the producers or green plants thus respire 56% of their gross production. Net ecosystem production is 422 dry g/m2/yr in this young forest. The ratio of total respiration to gross production is a convenient expression of successional status; a value of 0.82 for the Brookhaven forest indicates that this is a late successional community, but not in steady-state or climax condition (1.0). A leaf surface area of 3.8 m2 per m2 of ground surface intercepts sunlight energy, and the ratio of net primary production to incident visible sunlight energy gives a net efficiency of primary production of 0.0088. These and other functional characteristics of ecosystems are currently important topics of research—involving understanding of communities as biological systems, evaluation of the potential of environments to support life and man's harvest; and understanding of the fundamental meaning and consequences of man's alteration, exploitation, and pollution of ecosystems.