This article provides a contemporary overview of diagnostic models and systems for codifying mental disorders with four primary goals in mind: We first explain the purported essential purposes of using diagnoses in mental health care, including aspects of clinical management and communication with other professionals, service providers, patients and their relatives. Second, we highlight several important phases in the history of diagnostic classification covering the evolution from Kraepelinian descriptions to the current neo-Kraepelinian paradigm with all its unresolved issues. Third, we emphasize new insight into developmental and lifetime aspects of diagnoses, including issues related to diagnosing children and adolescents. Finally, we introduce novel scientifically informed diagnostic models, including the Hierarchical Taxonomy of Psychopathology (HiTOP) and dimensional classifications of personality disorders.