Since 1900, there has been 415 cases of chemical or biological terrorism in the U.S. and overseas. Since 1995, when the Japanese doomsday cult Aum Shinrikyo released sarin nerve gas on a Tokyo subway, terrorist incidents and hoaxes involving toxic agents have been on the rise. The Chemical and Biological Weapons Nonproliferation Project at the Monterey Institute's Center for Nonproliferation Studies compiled the database. The cases were divided into three categories: terrorist events, criminal events and state-sponsored assassinations. Since 1985, the number of terrorist incidents involving the threatened or actual use of chemical, biological, radiologic or nuclear materials has risen sharply. A 1995 peak was attributed with Aum Shinrikyo and copycat attacks. In 1998, actual-use incidents increased abruptly. Recent years have seen a rise in three types of terrorist organizations: single-issue groups, such as those dealing with abortion or animal rights; nationalist or separatist groups; and apocalyptic religious cults. Motivations, in descending order, are: to promote nationalist or separatist objectives; to retaliate for a perceived injury; to protest government policy; and to defend animal rights. The motivations have shifted over time. From 1975 to 1989, the predominant motivation was to protest government causes. Since 1990, the motivation has been to further nationalist causes or to seek revenge. Attacks with biological agents have been rare, with 66 criminal and 55 terrorist events between 1960 and 1999. Eight criminal attacks with biological agents led to 29 deaths and 31 injuries. Of the terrorist attacks associated with biological agents, only one resulted in casualties: the 1984 Salmonella bacteria contamination of salad bars by the Rajneeshee cult in Oregon. Few terrorist groups possess the capabilities to successfully release biological agents. The historical record suggests future incidents will probably involve hoaxes and small-scale attacks, such as food contamination. Tables, references