This thesis employs mixed methodologies, including systematic review and meta-analysis, qualitative research and pre-post design empirical testing. It comprises the in-depth formative work for designing and testing the Step-by-Step e-mental health intervention for depression. We found that a Western only model of mental distress does not capture important local features relevant to informing the design and conduction of efficacy studies of interventions for common mental disorders. We also found that local adaption of self-help psychological interventions can contribute to increased intervention efficacy. Systematically carrying out cultural adaptation of Step-by-Step with local actors was crucial. The feasibility testing of Step-by-Step provided valuable information to inform the design of future large-scale efficacy testing and hinted that Step-by-Step is a potentially promising intervention for reducing the symptoms of depression. An adapted, minimally guided, internet-delivered intervention like Step-by-Step could help close the treatment gap for depression if integrated responsibly into an existing care system.