The authors discuss the many sources of unreliability affecting the acquisition, retention, and retrieval of witnesses' material. These sources include stimulus events (e.g., exposure time, event complexity, event stressfulness), witness factors (e.g., expectations, information processing strategies), the length of the retention interval, distortion and changes in memory occurring over time, question biases, and lineup instruction biases. They also examine studies dealing with eyewitness activities related to suspect identification (e.g., photo spreads, mug shots, facial composites) and evaluate several possible means of improving or extending eyewitness performance, including use of different modes of questioning, hypnosis, and lie detectors. About 250 references are cited.