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The study evaluated the test-retest and interrater reliability of the Promoting the Emergence of Advanced Knowledge-Direct Training (PEAK-DT) assessment. In the study, the reliability of PEAK-DT scores were assessed both in terms of raw scores and age-referenced scores. To evaluate test–retest reliability, 39 participants with autism or a related disability were administered the PEAK-DT assessment on 2 occasions—during an initial assessment and a follow-up assessment after 3 weeks to 1 month. The results suggest that PEAK-DT had high test–retest reliability both for raw scores (intraclass correlation coefficient [ICC] = .987, p < .001) and age-referenced scores (ICC = .973, p < .001). In addition, 10 of the participants were assessed independently by 2 observers during the initial assessment period to evaluate interrater reliability. The results also suggested that PEAK-DT had strong interrater reliability (κ = .981, p < .001). Taken together, the results provide support for the reliability of PEAK-DT as a behavioral assessment of directly trained language skills.