The value of soybean seed [Glycine max (L.) Merrill] is derived primarily from the oil and protein that they contain. Seed composition can be modified to increase the seed's potential value, but these modifications often cause changes in productivity and yield. The individual seed is the fundamental unit responsible for the production of the oil, protein, and carbohydrates, and the processes governing the accumulation of these seed components and their relationships to productivity can be best understood by focusing on the individual seed. Oil and protein are synthesized in the seed from relatively simple building blocks (sucrose and a variety of amino acids) supplied by the mother plant. Much of the energy used for protein synthesis is expended by the mother plant (fixation of N2 and reduction of NO3-1), but the energy cost of oil production can be safely assigned to the seed. Seed composition could be controlled by the mother plant via the supply of raw materials or by the metabolism in the seed. In vitro studies with cotyledons suggest that genetic differences in seed protein levels are controlled by the seed. There is also evidence supporting regulation by the supply of raw materials, but the adjustment of seed number to the productivity of the plant community should minimize the variation in the supply to individual seeds. Variation in the duration of seed fill could also affect composition if the rate of synthesis of the individual seed components is not constant during seed filling. Environmental conditions that affect seed fill duration could, therefore, indirectly affect seed composition.