Major depression is a serious mental disorder that often takes a recurrent or chronic course causing enduring individual suffering as well as immense direct and indirect health costs. Research indicates that psychological continuation interventions following successful acute-phase therapy are effective in preventing depressive relapse and recurrence but access to these interventions is limited. Systematic psychological continuation interventions are hardly implemented in health care yet, and research shows that there are obstacles concerning access to and compliance for these interventions in a face-to-face setting underlining the need for alternative ways of delivery. The present study ("NaTel study") aims to investigate the effectiveness of telephone-administered cognitive-behavioral continuation therapy (T-CT) following acute-phase psychotherapy. The primary research question is whether participating in T-CT reduces depressive relapses within an observation period of 18 months compared with usual care alone. T-CT comprises eight therapy sessions delivered over the telephone by a trained therapist over a period of approximately six months after acute-phase therapy. Focus of the structured intervention is to train and foster relapse prevention strategies and to facilitate the transfer of skills acquired during acute-phase therapy to daily life. The effectiveness of T-CT as add-on to usual care is tested in a two-parallel group, multicenter, evaluator-blind clinical trial in patients with chronic/persistent or recurrent depressive disorder. Upon acute-phase therapy termination patients who have responded to cognitive behavioral therapy are randomized either to T-CT or usual care alone. Primary outcome of this study is relapse of a depressive episode. Relapse is determined by investigators blind to the study conditions based on clinical interviews conducted at months 6, 12, and 18 of follow-up. Further secondary outcome criteria are assessed with interviews and self-report questionnaires at various time points during follow-up. Overall, the study lasts approximately 36 months.