The rapid growth in access to and capabilities of digital health technologies presents a feasible route toward augmenting traditional mental health care and bridging the gap between the need for treatment and the capacity to deliver it. Smartphones provide a unique platform for fine-grained diagnosing, long-term symptom monitoring, and treatment. Digital phenotyping (both active [self-monitoring] and passive [sensor-based data]) refers to approaches in which personal data gathered from mobile devices and sensors is analyzed to provide health information on physiological functions and behavioral indicators. Smartphones have the ability to provide the efferent limb of intervention, which can, in turn, be optimally delivered by just-in-time intervention (JITAI) titrated through continuous feedback from digital phenotyping. Thus, this chapter will discuss the evidence available on the use of digital phenotyping for diagnosing and monitoring illness activity and the effect of smartphone-based treatment interventions in bipolar disorder.