Accurate measures of youth fitness are needed by researchers and practitioners. Evidence of validity and reliability are essential before results of youth fitness tests can be used to make sound decisions. This article describes a three-stage paradigm for validation research and provides guidance for conducting and understanding norm-referenced and criterion-referenced validity and reliability research. Advice is provided on how to administer fitness tests and how to use fitness test results in ways that promote reliability and validity in practice. Users of fitness tests are cautioned that interpretation and use of fitness tests involve important educational, pedagogical, and psychological consequences. Confidence in youth fitness test results and the decisions that are made based on these scores depend upon careful test design and administration that incorporate a sound understanding of the principles of validity and reliability. (Contains 1 figure and 4 tables.)