This chapter provides the readers with an understanding of traditional approaches to diagnosing and classifying psychopathology. It discusses different approaches to understanding, classifying, and diagnosing psychopathology in children and adolescents. The chapter also provides a summary of the limitations of each approach and introduces the benefit of a developmental psychopathology approach to conceptualizing psychopathology. Once mental health providers assess for the presence of psychopathological symptoms or observe certain behaviors in their patients, they can use a classification and diagnostic system to organize their findings and come to a differential diagnosis. The chapter reviews the history and content of two of the most well-known and widely used psychological disorder classification and diagnostic systems – the Diagnostic and Statistical Manual of Mental Disorders and the International Classification of Diseases. Hierarchical Taxonomy of The Psychopathology derived its psychopathology syndromes or disorders using statistical analyses, and is therefore a quantitative, “bottom up” approach to psychopathology.