This chapter discusses how the environment can affect decomposition. Starting first with temperature, the influences of heat and cold can alter the “normal” progress of a body breaking down. The effects of humidity and possible lack of oxygen are also described. The role of “big bugs” and “little bugs” is addressed, first beginning with forensic entomology. What and how insects, arachnids, and other arthropods can greatly influence the progression of decomposition is covered and the role of using forensic entomology to establish the time since death or postmortem interval is covered. The role of microorganisms in decomposition is covered including the thanatobiome or the postmortem microbiome. The presence and actions of different types of microbes are covered along with how decomposition is influenced by the presence of microbes. The process of submersion in water and drowning is covered along with basics of decomposition in water, where a body may be located, and how some drowning victims may appear. Finally, a photographic review of different stages contrasting pig carcasses placed in the winter and summer is provided for visual presentation of how decomposition may be influenced by weather.