The results indicate that the guilty knowledge technique cannot be used in the majority of FBI criminal investigations in which deception detection is used. The guilty knowledge technique should not be used as a general alternative to direct-questioning methods, however, where applicable, guilty knowledge techniques may be used as a supplement to other methods, possibly adding protection against false positives. The evaluation of the guilty knowledge technique involved a survey of 758 polygraph examination cases for operable case facts (referred to as “keys”). The case data of each criminal examination was collected by polygraph examiners from 25 FBI field offices from among cases assigned to them between November 1993 and August 1994. The sample comprised 35.6 percent of the criminal examinations conducted by the FBI during the study period. The operable case facts, or keys, were defined as a specific case fact that would likely be known to a guilty person and that, if not revealed, would likely not be known to innocent persons. A total of 355 operable keys were identified, which is an average of 0.47 operable keys per examination. Future research should focus on the applicability of guilty knowledge techniques in other law enforcement contexts, such as State and local investigations. Tables, references