Background: Acceptable, effective and feasible support strategies (interventions) for parents experiencing complex post-traumatic stress disorder (CPTSD) symptoms or with a history of childhood maltreatment may offer an opportunity to support parental recovery, reduce the risk of intergenerational transmission of trauma and improve life-course trajectories for children and future generations. However, evidence relating to the effect of interventions has not been synthesised to provide a comprehensive review of available support strategies. This evidence synthesis is critical to inform further research, practice and policy approaches in this emerging area. Objectives: To assess the effects of interventions provided to support parents who were experiencing CPTSD symptoms or who had experienced childhood maltreatment (or both), on parenting capacity and parental psychological or socio-emotional wellbeing. Search methods: In October 2021 we searched CENTRAL, MEDLINE, Embase, six other databases and two trials registers, together with checking references and contacting experts to identify additional studies. Selection criteria: All variants of randomised controlled trials (RCTs) comparing any intervention delivered in the perinatal period designed to support parents experiencing CPTSD symptoms or with a history of childhood maltreatment (or both), to any active or inactive control. Primary outcomes were parental psychological or socio-emotional wellbeing and parenting capacity between pregnancy and up to two years postpartum. Data collection and analysis: Two review authors independently assessed the eligibility of trials for inclusion, extracted data using a pre-designed data extraction form, and assessed risk of bias and certainty of evidence. We contacted study authors for additional information as required. We analysed continuous data using mean difference (MD) for outcomes using a single measure, and standardised mean difference (SMD) for outcomes using multiple measures, and risk ratios (RR) for dichotomous data. All data are presented with 95% confidence intervals (CIs). We undertook meta-analyses using random-effects models. Main results: We included evidence from 1925 participants in 15 RCTs that investigated the effect of 17 interventions. All included studies were published after 2005. Interventions included seven parenting interventions, eight psychological interventions and two service system approaches. The studies were funded by major research councils, government departments and philanthropic/charitable organisations. All evidence was of low or very low certainty. Parenting interventions. Evidence was very uncertain from a study (33 participants) assessing the effects of a parenting intervention compared to attention control on trauma-related symptoms, and psychological wellbeing symptoms (postpartum depression), in mothers who had experienced childhood maltreatment and were experiencing current parenting risk factors. Evidence suggested that parenting interventions may improve parent-child relationships slightly compared to usual service provision (SMD 0.45, 95% CI -0.06 to 0.96; I2 = 60%; 2 studies, 153 participants; low-certainty evidence). There may be little or no difference between parenting interventions and usual perinatal service in parenting skills including nurturance, supportive presence and reciprocity (SMD 0.25, 95% CI -0.07 to 0.58; I2 = 0%; 4 studies, 149 participants; low-certainty evidence). No studies assessed the effects of parenting interventions on parents' substance use, relationship quality or self-harm. Psychological interventions. Psychological interventions may result in little or no difference in trauma-related symptoms compared to usual care (SMD -0.05, 95% CI -0.40 to 0.31; I2 = 39%; 4 studies, 247 participants; low-certainty evidence). Psychological interventions may make little or no difference compared to usual care to depression symptom severity (8 studies, 507 participants, low-certainty evidence, SMD -0.34, 95% CI -0.66 to -0.03; I2 = 63%). An interpersonally focused cognitive behavioural analysis system of psychotherapy may slightly increase the number of pregnant women who quit smoking compared to usual smoking cessation therapy and prenatal care (189 participants, low-certainty evidence). A psychological intervention may slightly improve parents' relationship quality compared to usual care (1 study, 67 participants, low-certainty evidence). Benefits for parent-child relationships were very uncertain (26 participants, very low-certainty evidence), while there may be a slight improvement in parenting skills compared to usual care (66 participants, low-certainty evidence). No studies assessed the effects of psychological interventions on parents' self-harm. Service system approaches. One service system approach assessed the effect of a financial empowerment education programme, with and without trauma-informed peer support, compared to usual care for parents with low incomes. The interventions increased depression slightly (52 participants, low-certainty evidence). No studies assessed the effects of service system interventions on parents' trauma-related symptoms, substance use, relationship quality, self-harm, parent-child relationships or parenting skills. Authors' conclusions: There is currently a lack of high-quality evidence regarding the effectiveness of interventions to improve parenting capacity or parental psychological or socio-emotional wellbeing in parents experiencing CPTSD symptoms or who have experienced childhood maltreatment (or both). This lack of methodological rigour and high risk of bias made it difficult to interpret the findings of this review. Overall, results suggest that parenting interventions may slightly improve parent-child relationships but have a small, unimportant effect on parenting skills. Psychological interventions may help some women stop smoking in pregnancy, and may have small benefits on parents' relationships and parenting skills. A financial empowerment programme may slightly worsen depression symptoms. While potential beneficial effects were small, the importance of a positive effect in a small number of parents must be considered when making treatment and care decisions. There is a need for further high-quality research into effective strategies for this population.