PTSD is a mental disorder including psychological symptoms related to severe traumatic event(s). PTSD can negatively affect people's functioning in several life domains. Several effective therapies for the treatment of PTSD have been developed in recent decades. However, these treatments are not effective for every patient. Several studies show that people suffering from PTSD also experience feelings of trauma-related guilt, shame, self-blame and negative self-evaluation. These emotions may contribute to the maintenance cycle of PTSD or even become worse over time. In particularly in patients with trauma who have been exposed to repeated traumatic experiences within the context of interpersonal relationships including childhood sexual/physical abuse and domestic violence. This type of trauma is known as complex PTSD. cPTSD is marked by severe difficulties in problems with self and emotion-regulation, relationship difficulties and shame. One key factor for reducing self-criticism in individuals might be self-compassion as the antidote to self-criticism and shame. Compassion focused therapy by Paul Gilbert focuses specifically on increasing self-compassion. Although some studies show promising results of CFT in patients with cPTSS, the effectiveness for this group has not yet been sufficiently examined. Therefore, this Single case experimental design study is conducted with the primary objective of examining the effectiveness of CFT in reducing the primary outcome self-criticism. Secondary outcomes that will be examined are CFT reduces PTSD symptoms and shame and increases self-compassion and well-being. This SCED study is a noncurrent multiple baseline across subjects study, consisting of three phases with twice-weekly repeated measurements of self-criticism. First the pre-intervention baseline phase, in which participants are randomly assigned to different baseline lengths (either 5, 6 or 7 weeks). The second phase is the subsequent CFT-intervention, consisting of 12 weeks of weekly CFT group sessions with two-weekly assessments of self-criticism. The third phase is follow up for 5 weeks from the end of the intervention, again including two-weekly assessments of self-criticism. By comparing the baseline phases with the intervention and follow-up phases for individual participants, the effectiveness of the CFT intervention on self-critical beliefs can be determined. Changes that occur within participants can be seen as evidence of intervention effectiveness.