Conceptual issues about deceit, in specific why lies fail and when and how behavior may betray a lie, provides the basis for considering the type of experimental situations which may be fruitful for the study of deceit. New evidence, integrating past reports with new unpublished findings, compare the relative efficacy of facial, bodily, vocal, paralinguistic and textual measures in discriminating deceptive from honest behavior. The findings show also that most people do not rely upon the most useful sources of information in judging whether someone is lying.