Backgroung: Affective psychosis is a conceptual grouping used in clinical practice that lacks strong scientific basis. It includes patients who, in addition to psychosis, have a mood disorder. While early intervention has shown great efficacy, it mainly focuses on schizophrenia spectrum disorders, neglecting the affective forms of psychosis. Thus, this PhD thesis aims to study the relevance of the concept of affective psychosis in early intervention and its usefulness in developing stratification strategies. Method: These aims are addressed in two ways through four chapters, first through a literature review and then in the frame of three prospective studies conducted in a sample a first episode psychosis patients treated at the Treatment and early intervention in psychosis programme (TIPP) in Lausanne (Switzerland). The first chapter investigates literature on first-episode affective psychoses, outlining its specific clinical features and challenges. The second chapter explores the differences between affective and non-affective psychosis patients. The third chapter explores to which degree the various diagnostic categories included in the affective psychosis subgroup have sufficient commonalities to justify this grouping. Finally, the fourth chapter explores premorbid-based stratification strategies within affective psychoses to identify subgroups of patients that may require specific early intervention strategies. Results: Studies 1, 2, and 3 confirm the relevance of using the conceptual grouping of affective psychoses. Study 4 shows that the concept of affective psychoses regroups patients with various premorbid profiles that may require specific pharmacological and psychosocial treatment adjustment. Conclusion: This PhD thesis provides further evidence for the need of developing research in early intervention using the concept of affective psychosis, and points out clinical characteristics that may be used as therapeutic targets to develop adapted early intervention strategies for these patients.