22q11.2 Deletion Syndrome (22q11.2DS) is one of the strongest known genetic risk factors for schizophrenia and is a valuable model for understanding cognitive trajectories which may be associated with vulnerability for later psychosis. This thesis examined cognition and psychopathology over development in 22q11.2DS through cross-sectional and longitudinal approaches. First, in a large multi-site cross-sectional sample, I investigated whether the presence of Autism Spectrum Disorder (ASD), Attention Deficit Hyperactivity Disorder (ADHD) or anxiety disorder was associated with cognitive performance in children and adolescents with 22q11.2DS. In adults, I examined whether cognition was associated with presence of psychotic disorder. Psychopathology was associated with cognitive profile of individuals with 22q11.2DS in an age- and domain-specific manner. I also found that magnitude of cognitive impairment differed by developmental stage (child, adolescent or adult) in 22q11.2DS and the pattern differed by domain. Next, I longitudinally examined the trajectories of a range of cognitive domains over three timepoints in children and adolescents with 22q11.2DS as compared to a control group of siblings without the deletion. There was no evidence for cognitive deterioration, but mostly initial impairment which remained stable over time, or additional lags in some domains whereby individuals with 22q11.2DS were not progressing at the same rate as controls. Lastly, I compared the prevalence of prodromal psychotic symptoms in adolescents aged 15 years old with 22q11.2DS to control siblings, and whether longitudinal cognitive trajectories differed in individuals with 22q11.2DS and prodromal symptoms compared to those without. I found higher rates of prodromal symptoms in 22q11.2DS compared to controls, and that individuals with 22q11.2DS and prodromal positive psychotic symptoms displayed different antecedent cognitive development to those without psychotic symptoms in the form of initial deficits or lags over time. vi This thesis extends knowledge of cognitive development in 22q11.2DS and how this relates to psychopathology in both clinical and high-risk stages.