Psychologists are playing a new and increasingly important role in the courtroom by offering empirical information derived from psychological research. Social science involvement in the judicial process is illustrated by the landmark cases of Brown v. Board of Education and Ballew v. Georgia. In United States v. Amaral, the court believed that four criteria must be met for any expert testimony to be admissible. Analysis of these criteria leads to the conclusion that expert testimony of research psychologists on the issue of the credibility of eyewitness identification satisfies the law, as illustrated by a courtroom case. However, psychological testimony on the prediction of violent behavior in criminal and civil commitment cases has met with mixed legal reviews. Five other issues raised by the courtroom use of psychological data are internal validity, external validity, consensual validity, probabilistic conclusions, and the role of the psychologist's personal values. The internal validity of research findings should not be taken for granted; expert witnesses should be prepared to defend their research methodologies. The relevance of the research to the particular case should be addressed by the expert, who should inform the jury of any reasonable doubts about the applicability of the findings. Insufficient consensual validity of the data may require experts to discuss research evidence that opposes their testimony. Probabilistic research evidence should be presented as such; the jury should decide its applicability to a particular person. Expert psychologists should also be candid about their personal values so that the jury may evaluate any biasing effects on the testimony. It is concluded that difficulties involved with psychological testimony result from the clash of the differing traditions of law and psychology. Footnotes and references are provided.