The purpose of this study was to examine whether teachers’ motivation to diagnosing, attitude toward diagnosing, the self-efficacy, their knowledge, and reflection on experience in diagnosing would predict their diagnostic competence. The authors also examined whether teachers with professional experience were more competent diagnosticians than students in the second phase of German teacher education who in turn were expected to be more competent than students in the first phase. The findings demonstrate that teachers’ motivation to diagnose and teachers’ knowledge of diagnosing are substantial predictors of teachers’ diagnostic competence.