Internet-delivered programs based on cognitive behavior therapy (CBT) have during the past decade shown to work in an effective way for the treatment of depression. Due to its accessibility and independence of time and location, smartphone-based CBT might represent the next generation of digital interventions. Depression is an affective disorder that affects as many as 350 million people worldwide. However, with CBT, depression can be treated, but access to this treatment is scarce due to limited health care resources and trained therapists. As a result of this, health care could highly benefit from the use of smartphones for delivering cost-effective treatment that can be made available to a large part of the population who suffer from depression. One treatment that should be especially suitable for the smartphone format is behavioral activation (BA), since it has strong empirical support as well as the benefits of being flexible and rather simple.The overall aim of the thesis was to test and further develop a BA smartphone application, as well as to build a method for how this smartphone application could be used in a comprehensive and effective way in depression treatment. To fulfill this aim, four studies were conducted.Study I was a pilot study of the first version of the smartphone application, investigating how 11 participants experienced an intervention on a smartphone platform. The results showed initial implications that participants receiving the smartphone application used it in contexts where it would be difficult to use a traditional internet-based program.In Study II, the intervention was developed as a guided internet-based CBT intervention, but delivered through the smartphone application. The aim was to test the effects of this intervention compared to a smartphone-delivered mindfulness intervention. The results showed 5 large within-group effect sizes for both groups, but no between-group effect sizes. Subgroup analyses revealed a significant difference in favor of the BA program for participants with more severe depression at baseline. Contrary, for the participants with lower initial severity, the mindfulness program was significantly more effective than the BA program.Study III was an extension of the second study, using a qualitative approach investigating 12 participants' experiences of the intervention. One of the main findings was that the smartphone format seemed to be a portable and flexible way of accessing the treatment -and thus could be more present in everyday life.In Study IV, a blended treatment, using the BA smartphone application as an adjunct to four face-to-face sessions, was developed and tested against 10 face-to-face sessions. The results yielded large within-group effect sizes and no between-group effect sizes. This result is a preliminary indication that the number of face-to-face sessions can be reduced with the adjunct of a smartphone application.