affect eyewitness accuracy and are under the control of the criminal jus tice system have been labeled sys tem variables. Examples of system variables include instructions to eye witnesses, question formats, and the lineup procedure. Eyewitness researchers use slides, videotapes, or live staged events to create eyewitness experiences for unsuspecting people. These eyewit nesses can then be tested under con ditions controlling for system vari ables to determine the best ways to maximize eyewitness accuracy. The discovery of system variables that af fect eyewitness accuracy can yield a technology that can be applied to actual criminal cases for the purpose of minimizing eyewitness error.1 Our emphasis in this review is on police lineups, with particular con cern for the system variables that contribute to accurate and false identifications. Conducting a lineup is analogous to conducting a psychology experi ment.2 In a psychology experiment, the researcher begins with a hypoth esis, selects or creates an appropri ate set of stimuli, creates a design, places in motion a carefully crafted set of procedures, records the sub ject's responses, and reinterprets the hypothesis in light of the obtained data. A police investigator is like a researcher who has a hypothesis (that the suspect is the actual cul prit), selects an appropriate set of stimuli (selects lineup members), creates a design (e.g., places the sus pect in Position 4), initiates a proce dure (e.g., gives instructions to the eyewitness), records the subject's re sponses (e.g., identification of the suspect or not), and then reconsiders the original hypothesis in light of the