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This mixed-methods study examined how forensic evaluators define, interpret, and assess rational understanding in competency-to-stand-trial (CST) evaluations. Despite precedent from the Dusky v. United States (1960), rational understanding remains conceptually ambiguous, contributing to variability in CST opinions (Bonnie, 1992; Collins, 2019; Guarnera et al., 2017). Twenty-three licensed psychologists from the American Psychology-Law Society (Division 41) completed a survey evaluating their interpretations of rational understanding and responses to three brief CST vignettes. Quantitative analyses assessed whether vignette type, years of forensic experience, or number of completed CST evaluations influenced evaluator CST opinions. A chi-square test revealed a statistically significant difference in responses for Vignette 3 (p = .003), with a large effect size (Cramer’s V = .60). No significant correlations were found between evaluator experience variables and CST opinions. Qualitative thematic analysis revealed three main themes: (1) vignette-based decision-making, (2) interpretation of rational understanding, and (3) methodology for CST and rational understanding. Findings suggest that variability in how rational understanding is conceptualized, rather than differences in assessment tools, may underlie inconsistency in CST decisions. Although participants often reflected elements of established models (e.g., Bonnie, 1992; Grisso, 2003; Hoge, 2016; Rogers et al., 2003), their application varied, leading to discrepancies in opinions. This study recommended greater reliance on structured models, particularly Bonnie’s (1992) decisional competence and Grisso’s (2003) five-component model. In combination, these approaches may offer a more holistic and standardized method for assessing CST, especially rational understanding, thus improving conceptual clarity, interrater reliability, and fairness in CST evaluations.