This questionnaire is designed to assess the diagnostic competencies related to classroom-based assessments as a crucial component of data-based instructions. The target groups of the questionnaire are student teachers, trainee and pre-service teachers as well as teachers in the field of inclusive education. The questionnaire consists of four dimensions: (1) Instructional Decision-Making, (2) Educational Assessments, (3) Identification of Special Educational Needs, and (4) Progress Monitoring. The third dimension, Identification of Special Educational Needs, is specifically designed for special education teachers, while all other dimensions are applicable to all teachers. It is assumed that lessons in all types of schools are planned and designed in a differentiated and individually appropriate way for all students. A basic knowledge of the subject matter as well as diagnostic competencies are necessary in this respect, especially in inclu-sive and special schools, in order to make data-based instructions. The questionnaire provides a basis for assessing the diagnostic competence of teachers to plan further trainings and skill enhancement courses in diagnostics.