This book reviews the dimensional conceptualisation of persecutory delusion in the attempt to elucidate the transition from suspicious thinking to clinically relevant paranoia. For a better understanding of psychosis, a study of non-clinical population would prove useful: to this aim this chapter reviews some research works and proposed ideas. The book studies the epidemiological studies on psychotic features in the general population, and frames the findings in the dimensional and quasi-dimensional models of psychotic symptoms distribution.