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. The results showed that smartphones have the ability to be used in an effective way in the treatment of depression, including as an add-on to traditional face-to-face sessions. The results also showed that the smartphone format was experienced as a portable and flexible way of accessing the treatment – and thus could be more present in everyday life. In conclusion, there is reason to believe that smartphones will be integrated even further in society and therefore may serve an important role in future mental health care. Since the first indications reveal that depression can be treated by means of a supported smartphone application, it is highly possible that applications for other mental health problems will follow. Furthermore, in this thesis, the same smartphone application has been tested in three different ways and there is potential to apply smartphones in a range of other formats, such as in relapse prevention and as a way to intensify treatment during periods when needed. From a psychiatric research point of view, as my research group has been doing trials on guided internet treatment for more than 15 years, it is now time to move to the next generation of information technology – smartphones. / Internetbaserad kognitiv beteendeterapi (KBT) har under det senaste decenniet visat sig fungera effektivt i behandling av depression. På grund av dess tillgänglighet och flexibilitet i tid och plats har smartphonebaserad KBT potential att bli nästa generation av digitala behandlingar. Depression är en affektiv sjukdom som drabbar så många som 350 miljoner människor världen över. Med hjälp av KBT kan depression behandlas. Dock är tillgången till denna behandling knapp på grund av begränsade resurser i form av utbildade terapeuter. Därmed skulle sjukvården kunna dra stor nytta av användningen av smartphones för att leverera kostnadseffektiv behandling, som kan göras tillgänglig för människor som lider av depression. En behandling som kan vara särskilt lämplig för smartphone-formatet är beteendeaktivering (BA), eftersom den har starkt empiriskt stöd samtidigt som den är flexibel och relativt enkel. Det övergripande syftet med avhandlingen var att testa och vidareutveckla en smartphone-applikation baserad på BA, samt att bygga en behandlingsmetod för denna smartphone-applikation som skulle kunna användas för att effektivt behandla depression. För att uppnå detta syfte genomfördes fyra studier. Resultaten visade att smartphones kan användas på ett effektivt sätt i behandling av depression, bland annat som komplement till traditionella live-sessioner. Resultaten visade också initiala indikationer på att smartphoneformatet upplevdes som ett portabelt och flexibelt sätt att komma åt behandlingen - och därmed blev mer närvarande i vardagen.