Diagnostic competencies are now a key qualification for teachers. Only when teachers understand the strengths and weaknesses of their students can they provide individual support. The primary goal of this textbook is to present the scientifically grounded yet practical foundations of diagnostics that are essential for understanding and applying in the fields of school and teaching. The psychological theories that underpin diagnostic questions are discussed in detail. It is clearly demonstrated which diagnostic methods, whose results are truly relevant for teachers, can be used in the classroom. With this close alignment to the requirements of school and teaching practice, this volume is indispensable for training in all educational fields.