Staging has been increasingly used in unipolar depression since its introduction in the nineties. Several models are available, but their differential features and implications are not completely clear. We systematically reviewed: (a) staging models of longitudinal development of unipolar depression; (b) staging models of treatment-resistant unipolar depression; (c) their applications. MEDLINE, PsycINFO, EMBASE, and Web of Science were examined according to PRISMA guidelines from inception to December 2021. Search terms were: ‘stage/staging’, combined using the Boolean ‘AND’ operator with ‘psychiatric disorder/mental disorder/depressive/mood disorder’. A total of 169 studies were identified for inclusion: 18 described staging models or applications, 151 described treatment-resistant staging models or applications. Staging models of longitudinal development were found to play a key role in formulating sequential treatment, with particular reference to the use of psychotherapy after pharmacotherapy. Staging methods based on treatment resistance played a crucial role in setting entry criteria for randomized clinical trials and neurobiological investigations. Staging is part of clinimetrics, the science of clinical measurements, and its role can be enhanced by its association with other clinimetric strategies, such as repeated assessments, organization of problematic areas, and evaluation of phenomena that may affect responsiveness. In research, it may allow to identify more homogeneous populations in terms of treatment history that may diminish the likelihood of spurious results in comparisons. In clinical practice, the use of staging in a clinimetric perspective allows clinicians to make full use of the information that is available for an individual patient at a specific time.