This chapter applies the Standards for Educational and Psychological Testing as a framework for understanding the test validity of psychological measures. It also focuses on several Supreme Court decisions that address the Daubert standard and the pivotal role of measurement error in forensic assessments. The next major section focuses on the development of psychometric standards with recommendations for standardized terminology. Using effect sizes (Cohen’s d and Hedge’s g) as a salient example, the following clinical criteria are proposed: “very large” (d ≥ 1.50), “large” (d ≥ 1.25), “moderate” and “insubstantial" (d < 0.75). Similarly, clinical criteria are implemented for alphas, interrater reliabilities, and measurement errors. The final major section addresses to two diagnoses which are central to psychological injuries: posttraumatic stress disorder (PTSD) and mild traumatic brain injury (mTBI).