The ecosystem concept has been particularly useful and extensively employed in the study of aquatic primary productivity. The flow of energy through the system is an attractive area of investigation when it involves some process, but has a more restricted value when units of biomass are simply converted to calories. Although we are able to measure primary productivity in terms of the carbon fixed, we are not yet able to measure the actual change in the oxidative state of the newly fixed carbon. The fate of photosynthate as food for higher trophic levels is therefore dependent upon a considerable array of biological and environmental variables. Primary productivity is considered in terms of its evolution from measures of standing crop and yield, which have been gradually replaced by measures of rate of carbon uptake or oxygen production, or by measure of nutrient loss, or by change of CO2 in the environment. Data from five lakes are used to illustrate the evolutionary thread of eutrophication and the great range in primary productivity to be expected on the basis of either unit volume or unit surface area at different trophic states. Light and nutrients are important in limiting primary productivity, and are contributing factors to the great variability which one may encounter within a given lake. Only with a sounder understanding of productivity at the base of the food-chain can we have any real hope of controlling the productivity of aquatic environments for the benefit of man.