The results of a lineup procedure can be used as evidence in court only if the witness recognizes the suspect exclusively because of the similarity between the person identified in the lineup and the mnemonic representation of the person seen before in the situation of the criminal act. The article examines some of the errors and reports on hypotheses derived from assumptions about lineup procedures: (1) Suspects differ in their psychological states from non-suspects used as distractors (usually policemen) in real lineup procedures; (2) Suspects' behavior in a real lineup is perceived as more authentic than the behavior of persons used as distractors; (3) It is possible for non-witnesses to identify a suspect in a lineup procedure although they have never seen him before because of his different psychological state and his greater behavioral authenticity revealed by non-verbal cues; (4) Suspects who are merely asked whether or not there is a suspect in the lineup will more often answer "I don't know" than those who are told there is a suspect in the lineup; and (5) Subjects who are told there is a suspect in the lineup will more often choose the target person than subjects who are only asked whether or not there is a suspect in the lineup. Tables, figures, references