The primary objectives in biological wastewater and sludge treatment processes are to procedure a minimum quantity of solid, stable residues and a maximum quantity of carbon dioxide from the organisms present in the process feeds. As far as minimizing solids production in activated sludge type processes is concerned, endogenous activity in the recycled biomass is usually considered to be the most important mechanism. However, increased understanding of the growth characteristics of mixed microbial cultures suggests that lysis and “cryptic” growth are probably dominant mechanisms. For pathogen destruction in treatment processes, death and subsequent lysis of pathogens are clearly events that must be promoted. Here, the kinetics for death, lysis and “cryptic” growth in aerobic wastewater treatment and aerobic sludge stabilization processes are examined.