Paranoia, or persecutory delusions, is a quintessential symptom of psychotic disorders such as schizophrenia. Individuals suffering from paranoid ideations become increasingly isolated, avoidant of social situations, and unhappy. These unfounded fixed suspicions that others are out to harm the individual exist on a continuum of severity across clinical and community populations, with 1–3% of nonclinical populations having delusions of clinical severity, a further 5–6% having a delusion but of less severity, and 10–15% reporting regular paranoid thoughts. This dimensional approach has recently been applied to children and to groups from different countries. Much progress has been made on the causes and treatments of paranoia, and these remain significant areas of research and clinical interest. Understanding paranoia and its correlates developmentally continues to be critical to our understanding of the etiology of schizophrenia-spectrum disorders.