Publisher Summary Bacteria in a state of active growth and division present interesting problems of regulation. The respiration rate must be appropriate to the conditions of growth in an environment subject to rapid and radical changes. This chapter attempts to rationalize the available date relevant to understanding the type of regulatory mechanisms involved in control of respiration in growing bacteria. Bacteria growing on a compound that serves as both carbon and energy source are faced with the problem of apportioning the substrate between catabolic and anabolic processes. The main difference in regulatory mechanisms between growing and non-growing states is that the growing state allows for responses, which involve changes in the enzyme constitution of the bacteria, while induction and repression of enzyme synthesis are unlikely to contribute to regulation in non-growing cells. Otherwise, the differences among the more rapidly-responding regulatory systems (i.e. those not involving repression and induction of enzymes) in growing and non-growing bacteria will be a result of differences in the environment and physiological state.