The mindfulness-based intervention consists of three five to ten minutes session-introducing interventions (mindful walking, body scan, breathing space). At the beginning of each of the 24 therapy sessions patients receive one of the three mindfulness interventions. Each intervention is instructed for four sessions consecutively and eight sessions in total. After completion of the mindfulness intervention, the regular therapy session begins.

The relaxation interventions (progressive muscle relaxation (PMR), imagery journey, walking relaxation) are parallelized to the three mindfulness-based interventions. At the beginning of each of the 24 therapy sessions, patients receive one of the three relaxation interventions. Each intervention is instructed for four sessions consecutively and eight sessions in total. After completion of the relaxation intervention, the regular therapy session begins.

Standard cognitive behavior therapy treatment, based on the individualized case conception of the trainee therapist, is conducted during the whole treatment sessions. No particular session-introductions are applied.

In all three treatment arms, trainee therapists perform a cognitive behavior therapy (CBT) under conditions of the German health care system. This treatment is not a manualized intervention, but rather based on individualized treatment plans that have been developed together with expert supervisors during a five session diagnostic stage. Treatment duration is 24 sessions, while on average every fourth is supervized by an CBT expert therapist.

The YSR 11-18 R measures general symptom severity in patients. Eight different subscales are differentiated across 112 items: anxious/depressed, withdrawn/depressed, somatic complaints, social problems, thought problems, attention problems, delinquent behavior, and aggressive behaviors. The three subscales anxious/depressed, withdrawn/depressed and somatic complaints are regarded as internalizing problems while the two subscales delinquent behavior and aggressive behavior are regarded as externalizing behavior. To calculate subscales as well as the total score corresponding items are summed. Items are rated on a 3-point Likert-scale (0 = not true, 1 = somewhat or sometimes true, 2 = very true or often true). Higher scores reflect higher symptom severity. T-scores are used to interpret and compare the results to the corresponding age and gender group.

The FTB-KJ is a self-report instrument to measure therapeutic alliance. It contains a patient and therapist perspective with 12 items rated on a four-step scale. In addition to the items of the FTB-KJ, a two-item short version of the Therapeutic Presence Inventory will be applied. Furthermore, two items will assess if patients and therapists perceive the session-introducing intervention positively or negatively.

The six items of the PQ-M assess the perceived quality of mindfulness implementation operationalised as perseverance in (a) receptive (b) present-moment attention.