This paper reviews the four types of validity that make up Cook and Campbell's traditional approach for social science research in general and psychological research in particular: internal validity, statistical conclusion validity, external validity, and construct validity. The most important generalizability threat to the validity of jury research is not likely a selection main effect (i.e., the effect of relying solely on undergraduate mock jurors) but is more likely the interaction of sample with construct validity factors. Researchers who try to capture the trial process with experimental paradigms may find that undergraduate mock jurors react differently to those efforts than do more representative community samples. We illustrate these issues with the seven papers that make up this volume, and conclude by endorsing Diamond's call for a two-stage research process in which findings with samples of convenience gradually add more realistic trial processes and representative samples to confirm the initial findings and increase the research program's credibility. Copyright © 2011 John Wiley & Sons, Ltd.