This chapter discusses thanatology, scientific study of death in all its aspects including its cause and phenomena. Brain/brainstem death, modes of death, sudden death, anoxia, and coronary atherosclerosis, are some topics discussed in detail. In brain death, increase in the intracranial pressure compresses the entire brain, including the brainstem, and total brain infarction follows. Modes of death include coma, syncope, and asphyxia. In cases of sudden death, it is usually not possible to ascertain the cause of death from an external examination of the body. Causes of sudden death can be related to cardiovascular, respiratory system, central nervous system, gastrointestinal system, genito-urinary system, reproductive system, endocrine, or iatrogenic. Most anaphylactic deaths seen by forensic pathologist are caused by insect bites, drugs, or foods. Some individuals show marked hypersensitivity to stimulation of the carotid sinuses, characterized by bradycardia and cardiac arrhythmias ranging from ventricular arrhythmias to cardiac arrest.