This chapter presents an overview of energy release and conservation mechanisms beginning with glucose degradation to pyruvate. Fermentation, aerobic respiration, and anaerobic respiration are then examined. The consideration of chemoorganoheterotrophic metabolism concludes with a discussion of the catabolism of lipids, proteins, and amino acids. Chemolithotrophic metabolism follows and the chapter concludes with a discussion of the trapping of energy by photosynthesis.