Estimation of time of death is important evidence in a homicide case, because it can corroborate or disprove a suspect's alibi regarding where he/she was at the time of the victim's death. Over the years, forensic scientists have attempted to develop a definitive method for determining time of death, but no single reliable method has been developed. Moreover, it is impossible to fix the exact time of death because of the many variables that influence the rate of a corpse's decomposition. The best estimate of time of death by an experienced analyst will be within a range of hours. In estimating time of death, it is important that the first trained investigator who observes the body record its condition and the characteristics of the environment that can influence the rate of the body's decomposition. After explaining the biological processes that sustain human life, this article describes what happens in the body from the moment these life-giving processes cease at death. Physical symptoms of the stages of decomposition are described, including skin color and body heat, rigor mortis and livor mortis, insect activity in and around the body, stomach contents of the body, and putrefaction (decomposition). Other crime-scene information pertinent to time of death is also discussed. Photo illustrations