Feelings of loneliness are common in persons with mental health problems. Studies indicate that loneliness is related to both adverse somatic and psychiatric outcomes. Longitudinal examinations also suggest that loneliness may contribute to increased and maintained symptoms of depression and social anxiety. Even if there is no gold-standard intervention for helping persons experiencing clinically significant levels of loneliness, cognitive behavior therapy (CBT) has been suggested as an option. The interest in CBT partially stems from earlier trials, but also from studies investigating cognitive and behavioral correlates of frequent feelings of loneliness. In this chapter, we outline suggestions for developing and disseminating internet-based interventions targeting loneliness based on our experiences from developing a treatment and testing it in two randomized controlled trials. The initial results suggest that internet-based CBT (ICBT) can be a feasible approach to alleviate clinically relevant feelings of loneliness.