Unconditional Self-Acceptance (USA) allows for rating one’s attributes, abilities, and behaviors but not one’s entire personhood. High USA may be distinguished from high self-esteem, or any level of self-esteem, in that people with high USA do not infer their general level of worth from how they are currently faring in meeting their own goals or others’ expectations. The relevance of USA to psychological health is emphasized in rational emotive behavior therapy (REBT). This chapter describes the development and validation of a measure of USA as conceptualized in REBT. The Unconditional Self-Acceptance Questionnaire (USAQ) is a 20-item questionnaire that has been translated into several languages. The USAQ is internally consistent, and several lines of research support its concurrent validity. It is positively correlated with mindfulness measures. More research is needed on its retest reliability, sensitivity to treatment (including mindfulness) interventions, factor structure, and discriminant validity. An especially important open question is the empirical distinguishability of USA from self-esteem.