This chapter reviews the historical development of the construct of schizophrenia as applied to both adults and children. It presents some of the basic tools of clinical epidemiology which can help to quantify diagnostic certainty in children and adolescents. The chapter compares diagnostic and statistical manual of mental disorders (DSM)-IV and International classification of diseases (ICD)-10 diagnostic criteria, and examines the implications for their different diagnostic thresholds when applied to children and adolescents. It considers the use of assessment instruments, in particular the potential for using symptom rating scales to improve diagnostic accuracy. The chapter provides the evidence for the validity of adult diagnostic criteria in children and adolescents. It considers how the pattern of premorbid and psychotic symptoms differs between childhood onset and adult onset schizophrenia. The chapter describes the process of differential diagnosis psychotic symptoms in children and adolescents, and discusses the potential for prevention and early diagnosis.