Supported treatments are not broadly available, prompting the need for effective training of front-line providers. Observational methods for assessing training success are resource- and time-intensive, and brief self-reports of comprehension are prone to bias and poor correspondence with practice. The present study presents a preliminary psychometric evaluation of a novel alternative method to assessing trainee comprehension of and clinical reasoning in cognitive–behavioral therapy (CBT) for child anxiety disorders, the Assessment of Clinical decision-making in Evidence-based treatment for Child Anxiety and Related and Disorders (ACE CARD). The ACE CARD, developed in consultation with a panel of child anxiety experts, presents respondents with clinical vignettes describing prototypical anxious youth at various points of CBT treatment and has respondents select which of 4 response options is most consistent with the CBT model of child anxiety treatment. In a sample of novice trainees and experts in CBT for anxiety (N = 54), results suggest the measure can discriminate across levels of clinical expertise. Experts performed significantly better than trainees (d = .60, p < .001), although 3 of the vignettes were largely responsible for group differences. Future iterations of the ACE CARD are needed to remove items with poor discriminating properties and to add additional items with greater difficulty. With continued support, vignette-based assessment approaches may prove to have a meaningful role in dissemination efforts as brief, resource-efficient proxies of CBT competence.