The extreme suffering, personal and economic costs of major mental disorders have focused many research efforts on the identification of vulnerability factors for the development of these disorders. Vulnerability factors are those characteristics that increase the probability that an individual develops a disease and are present before the onset of the disorder. The identification of these characteristics in the general population would permit the final objective of early detection of mental disorders. Longitudinal high-risk studies are the most important and complex method of early detection. These studies perform a follow-up of individuals who still do not present the disorder but show a putative vulnerability marker for a certain mental disorder. In this book, we present a longitudinal study of sustained attention across adolescence and analyse its possible role as a vulnerability marker for schizophrenia spectrum disorders. These data shed some light on the field of early detection of psychosis and may be useful for mental health professionals who want to extend their knowledge on the dimensions of premorbid risk for mental illnesses, especially psychotic disorders.