Publisher Summary This chapter examines the factors that affect the composting process. In composting, the substrate is the waste to be composted and, similar to any other biological process, the chemical and physical characteristics of the substrate are critical in the viability of the process. The complexity and nature of the molecular structure of the substrate are particularly important because these characteristics define the assimilability of nutrients by various microorganisms. Composting is a bio-oxidative microbial degradation process of mixed organic matter. This exothermic process produces a relatively large quantity of energy. High pH values in the starting material in association with high temperatures can cause a loss of nitrogen through the volatilization of ammonia. Water is essential for all microbial activity and should be present in appropriate amounts throughout the composting cycle. Optimal moisture content in the starting material varies and essentially depends on the physical state and size of the particles and on the composting system used.