Hermann Emminghaus was the first to introduce a developmental perspective into child psychiatry, with special focus on psychoses. This chapter describes the general criteria for the classification of psychotic disorders in children and adolescents. In childhood and adolescence, however, age and developmental stage play a very important role in the classification of schizophrenia. The chapter discusses the psychotic disorders in childhood and adolescence and their relation to schizophrenia. Bettes and Walker found a strong effect of age on the manifestation of positive and negative symptoms. Positive symptoms increased linearly with age, while negative symptoms occurred most frequently in early childhood and late adolescence. About 50% of children and adolescents with schizophrenia show an uncharacteristic symptomatology in their premorbid personality. The chapter explains the classification of schizophrenia and other psychotic disorders according to International Classification of Diseases (ICD)-10 and Diagnostic and Statistical Manual of Mental Disorders (DSM)-IV.