Major depression is expected to be the disorder with the highest disease burden in high-income countries by the year 2030. The efficacy of behavioural activation in the treatment of major depressive disorders has been established in a number of studies over the last four decades. Furthermore, behavioural activation is an intervention that can largely benefit from the use of new mobile technologies (e.g. smartphones). One important feature of mobile technology is the possibility for the therapist to reach the patient between sessions and thus create direct incentives for behavioural activation in everyday life. Therefore, developing smartphone-based behavioural activation interventions might be a way to develop a cost-effective treatment for people suffering from major depressive disorders.