Atopic eczema (AE) and psoriasis are common inflammatory skin diseases associated with substantial morbidity for affected individuals. Evidence suggests associations between AE or psoriasis, and depression, anxiety, and severe mental illness (SMI). However, there is limited evidence on: (1) factors associated with the mental health conditions among those with AE or psoriasis; (2) longitudinal associations between AE or psoriasis, and SMI; and (3) associations between AE and depression and anxiety in different ethnic groups. This thesis aims to: (1) systematically review available evidence on factors associated with depression, anxiety, and SMI among adults with AE or psoriasis; (2) investigate longitudinal associations between AE or psoriasis and incident SMI among adults using electronic health records from the UK’s Clinical Practice Research Datalink (CPRD); and (3) investigate whether associations between AE and incident depression or anxiety differs between adults from white and minority ethnic groups using CPRD data. The systematic review included 21 studies and identified factors that were associated with depression (being female, psoriatic arthritis) and anxiety (being female, psoriatic arthritis, moderate-to-severe psoriasis) among adults with psoriasis. Evidence from included trials suggested AE or psoriasis treated with placebo was associated with higher depression and anxiety than targeted biologic treatment. AE and psoriasis were associated with increased SMI risk, and among those with AE, the increased risk was largely mediated by problems with sleep. Adults with AE were also at increased risk of depression and anxiety. Depression and anxiety risk was higher in individuals from minority ethnic groups compared to those of white ethnicity. In conclusion, evidence from this thesis suggests a large burden of mental health conditions among those with AE or psoriasis. The introduction of mental health promotion and prevention strategies in the management of individuals with AE or psoriasis may improve health outcomes and reduce mental health burden.