This chapter analyzes the main components of the problem and draws broad conclusions regarding the relationships between energy-generating and energy-consuming reactions extant in aerobically growing heterotrophic organisms. Growth involves the uptake and chemical transformation of a small number of substances, the energy for this process being derived, in heterotrophic organisms, from the oxidation of the carbon substrate. All living organisms seemingly contain carbon, hydrogen, oxygen, nitrogen, sulphur, and phosphorus, as well as potassium, magnesium, and a number of so-called ''trace'' elements. At low growth rate values respiration was severely impeded, and much acetate and D-lactate were excreted. Fundamental to the interpretation of yield values expressed by carbon-substrate-sufficient chemostat cultures is the extent to which the uptake of carbon substrate can be modulated when growth is limited by the supply of some other essential nutrient. With carbon-substrate-limited chemostat cultures, the substrate uptake rate declines to a small, but finite, value at zero growth rate.