Algorithm for the Retrospective Diagnostic Assessment of ICD-10, ICD-11, and DSM-5 ADHD Symptoms in Primary School Reports Abstract: According to the German S3 guideline, diagnosing attention deficit/hyperactivity disorder (ADHD) requires evidence of symptom onset during childhood. This poses a particular challenge when assessing adolescents and young adults. Primary school reports can provide valuable information about earlier patterns of work and social behavior, thereby allowing a retrospective assessment of ADHD-related symptoms - specifically inattention, impulsivity, and hyperactivity. In our 2025 study (Waltereit et al., 2025), we systematically and quantitatively analyzed primary school reports from a sample in the German federal state of Saxony using the diagnostic criteria for ADHD according to ICD-10, ICD-11, and DSM-5. The results demonstrated high diagnostic discrimination between reports of children with ADHD and those of healthy controls. Based on these findings, we developed the ADHD-ZEBRA (Analysis of Primary School Reports for Symptoms of ADHD) through an iterative design process. This article presents and discusses the procedure that operationalizes the empirical findings for the practical application in clinical settings. The aim is to make this frequently used, but to date nonstandardized source of information more usable and to strengthen the diagnostic validity of ADHD in adolescence and young adulthood.