Having a child diagnosed with cancer is stressful and many parents of children on treatment for cancer report symptoms of posttraumatic stress (PTSS). The overall purpose was to, among parents of children on treatment for cancer, investigate the factor structure of PTSS; investigate the relationships between experiential avoidance (EA), rumination, PTSS and depression; and to develop, test, and evaluate a guided self-help intervention provided via the internet. In a longitudinal study with three assessments (n = 249-203) results indicated that a four-factor solution of PTSS including the factors re-experiencing, avoidance, dysphoria, and hyper-arousal provided best fit and that the pattern and size of factor loadings were equivalent across the three assessments (Study I). In a case study with pre-, post-, and follow-up assessments a guided self-intervention was well received with clinical significant and reliable improvements in PTSS, depression, and quality of life (Study II). Furthermore, in cross-sectional analyses (n = 79) EA and rumination were positively associated with PTSS and depression and provided incremental explanation in depression while controlling for demographic characteristics, anxiety, and PTSS. In longitudinal analyses (n = 20), EA but not rumination predicted PTSS and depression while controlling for initial levels (Study III). Finally, in a randomized controlled trial with parents fulfilling the modified symptom criteria on the PTSD-Checklist allocated to guided self-help via the internet (n = 31) or to a wait-list control condition (n = 27) there was a significant intervention effect with a large effect size for the primary outcome PTSS. Similar results were observed for the secondary outcomes depression and anxiety, but not for EA and rumination. Exploratory analyses suggested that the relationships between EA and PTSS and between EA and depression were weakened in the intervention group (Study IV). The studies included in the current thesis suggest that a four-factor solution should be used when assessing PTSS in parents of children on cancer treatment. Furthermore, rumination and EA in particular seem to be important constructs to consider when understanding PTSS and depression in this population. Finally, guided self-help via the internet shows promise in reducing PTSS and depression among parents of children on cancer treatment who report a high level of PTSS.