The Instruments for Assessing Understanding and Appreciation of Miranda Rights, hereafter referred to as the Grisso tests, are widely used and recommended psychological tests designed to help assess ability to understand and appreciate Miranda warnings. This article describes the results of these tests administered to 439 individuals who participated in an evaluation to assess knowing and intelligent waiver of Miranda warnings during police questioning. This study is the first of its kind using a large sample of defendants who were referred for an assessment of capacity to waive Miranda rights. This forensic sample includes both juveniles (176) and adults (263), and the results are quite similar to the original Grisso norms. There are strong correlations between performance on these tests and IQ as well as between performance and age. This sample provides a broader foundation for users of the Grisso tests and answers some of the initial criticism of the instruments. Copyright © 2012 John Wiley & Sons, Ltd.