Litigation against police agencies for the use and misuse of control agents and procedures has become an important issue for the forensic medical specialist and expert witness. As police technology improves, it becomes apparent that more information for forensic experts will be required in this area. One such area of concern is the use of tear gas by police. This paper addresses the basic types of gas in terms of their chemistry, uses, and medical side effects and complications. Two types of gases--namely, chloroacetophenone (CN) and orthochlorobenzylidenemalontrile (CS)--are discussed in terms of basic properties, unique advantages, and possible harm to the body. CN and CS are relatively safe agents that can be used to control the behavior of a dangerous person. Clinical examples of their use and misuse by police are also reported. Specific guidelines are offered to help ensure a proper assessment of the dangerous person and to protect the health and lives of police personnel, perpetrators, and others exposed to the tear gas. Forensic experts called to testify about the behavior of the disturbed person or to assess the appropriateness of action taken to subdue that person must become more aware of the technology employed by police agencies to accomplish this particular job of protecting public safety.