The competence of young children to testify as witnesses in a court of law, especially with regard to the obligation to speak the truth, is examined. The relevant empirical data from developmental literature is critically reviewed. Topics considered include the development of the concept of lies and truth, moral development, the relationship between moral reasoning and moral conduct, and observations of moral behavior. The ecological validity of laboratory analogs is considered in light of the extreme sensitivity to context of children's abilities and behavior. From available evidence, we conclude that young children consistently judge lying solely on whether what is said agrees with external facts regardless of inferred intent or belief of the speaker. Further, young children are less willing to lie because of extenuating circumstances than are older children and adults. The older child's and adult's decision on whether to tell a lie is based on a consideration of a number of complex internal and environmental factors. We conclude that children on the witness stand are likely to be motivated to be as honest, or more honest, than are adults.