Postpartum depression is a psychiatric disorder affecting a substantial proportion of newly delivered women, and remains a significant cause of childbirth-related morbidity. The aim of the present thesis was to examine psychological, endocrine and genetic aspects of postpartum depression in a large, population-based sample of women in Uppsala, Sweden. All included studies were undertaken as parts of the BASIC-project, a longitudinal study on psychological wellbeing during pregnancy and the postpartum period. Study participants were screened for depressive symptoms in pregnancy week 17 and 32 as well as at six weeks and six months postpartum, mainly by use of the Swedish version of the Edinburgh Postnatal Depression Scale (EPDS). Furthermore, personality was assessed with the Swedish universities Scale of Personality (SSP) in pregnancy week 32. Evening cortisol levels in saliva were measured in pregnancy week 36 and at six weeks postpartum. Blood samples were obtained to measure corticotropin-releasing hormone levels (CRH) and to perform genetic analyses. The results of this thesis demonstrate that neuroticism is a strong and independent predictive factor of depressive symptoms at six weeks and six months postpartum, and has a significant mediatory role in the association between a single nucleotide polymorphism in the hydroxysteroid (11-beta) dehydrogenase 1 gene (HSD11B1) and postpartum depression. Furthermore, women with postpartum depressive symptoms present with a dysregulated hypothalamic-pituitary-adrenal axis activity in terms of elevated cortisol levels postpartum, as well as elevated CRH levels in mid-gestation. In conclusion, this thesis develops current knowledge on several attributes of postpartum depression. Further studies are required to replicate and expand on these results, which would further contribute to early identification of women at risk of postpartum depression and adoption of proper interventions that may moderate the short- and long-term consequences of the disorder.