The analysis of the suspect's decisionmaking has important implications for the legal concept of voluntariness, that all decisions made during interrogation must not be significantly affected by external sources of interference. Three areas where the interrogator controls the flow of information may produce significant interference with decisionmaking: (1) what happens to the suspect, such as the length of detention and what charges are filed; (2) offering the suspect the chance to win or lose social approval or self-esteem; and (3) providing or withholding information about which the suspect is ignorant but the interrogator is an expert, such as probable length of sentence. Interrogators can manipulate these forces in such a way that they destroy a statement's voluntariness. Moreover, factors which affect voluntariness also influence reliability. Situational stresses likely to impair a subject's decisionmaking during interrogation arise from three sources: physical characteristics of the suspect's environment in the police station; confinement and isolation of the suspect from his or her peers; and the subject's submission to authority. While empirical research is not conclusive, it suggests that impairment can be an issue when an interrogation occurs after midnight, when the suspect has lost more than 5 hours sleep, and when the interrogation continues for more than 1 hour without a break. The interrogator must pick up and interpret information coming from the suspect, as well as manage his or her own behavior by projecting authority, controlling the information flow, and manipulating the suspect's view of the options. Training is no substitute for experience, but it can facilitate learning interrogation skills, minimize bad habits, and enhance natural skills. The paper discusses such training in the San Francisco Police Department (California) and two major technical aids available to interrogators -- the polygraph examination and hypnosis and hypnotic drugs. Approximately 190 references are provided.