Fixed, false beliefs—known as delusions—are a hallmark of psychotic illness. While considerable research has explored the emergence of delusions, relatively little has focused on their fixity. Here we recap classical and contemporary descriptions of delusions and highlight some outstanding issues besides fixity. We then examine the definition and measurement of belief fixity to date and propose a clearer vocabulary for the fixity of delusions in particular, in terms of their conviction, incorrigibility, persistence and stability. Using these terms, we review the extent to which delusions may truly be considered fixed and the diagnostic, psychopathological, psychodynamic, social, cognitive, metacognitive and cognitive neuroscience factors influencing this. We then integrate this evidence in a working model of delusion fixity. Finally, we summarize limitations of the existing literature and highlight opportunities for future research. We predict that longitudinal within-patient studies, which work across specialisms and perspectives, will be particularly valuable for furthering an integrated understanding of delusion fixity.