The Belief Formation Program investigated the processes that underpin how we perceive reality, generate beliefs, monitor and evaluate evidence, and adopt and revise beliefs when needed. Disruptions to these processes can cause psychotic symptoms (i.e., delusions and hallucinations) that characterise psychiatric illnesses such as schizophrenia, and present in other clinical conditions (e.g., dementia). Our aim was to develop cognitive and neural models of these symptoms across diagnostic categories, advance understanding of related non-clinical phenomena, develop and evaluate psychological treatments, and examine social influences and consequences.