Parental substance misuse is a pervasive risk factor for a range of detrimental outcomes for both children and parents across the life course. While many interventions have been developed for this population, the existing evidence-base requires consolidation and consideration of the comparative effectiveness of different interventions to facilitate evidence-informed decisions between different intervention approaches. There is a need to also understand the therapeutic processes of change to inform efficacious treatment implementation that is more likely to generate desired outcomes for families. Therefore, this dissertation first provides a systematic review and comparative synthesis on the effectiveness of psychosocial, legal, and pharmacological interventions for improving outcomes for children with substance misusing parents. Second, this dissertation examines the therapeutic process of change for a cohort of substance misusing parents who engaged in an intervention that aimed to improve child, parent, and overall family wellbeing. Study 1 identified 85 evaluations of psychosocial, legal, and pharmacological interventions for families affected by parental substance misuse. These interventions were evaluated using a large range of child psychosocial outcomes which broadly fell under: (a) child welfare; (b) child development; (c) child emotional and behavioural; and (d) educational domains. Due to a large amount of missing data, many effect estimates were underpowered and based on either single studies or meta-analyses with very small sample sizes. Nevertheless, the review found that integrated psychosocial interventions – which may or may not contain pharmacological or legal components – can reduce child abuse potential, the likelihood of out-of-home care, and overall child emotional and behavioural difficulties in the short-term. They can also increase the odds of parents reunifying or retaining care of their children, enhance expressive language development, and increase child prosocial behaviours in the short-term. There is only preliminary evidence that effects are maintained within six to 12 months post-intervention for total child emotional and behavioural difficulties, and some indicators that effects may emerge at these later time-points for reunification, retaining care, and permanency. Study 2 examined the interactive process of change in parent psychopathology and mindful parenting during participation in the Parents under Pressure (PuP) program for a cohort of 166 families affected by parental substance misuse. The community-based evaluation was implemented across 11 service sites in the United Kingdom. Baseline, midtreatment, and end-treatment assessment of parent psychopathology and mindful parenting was obtained. Cross-lagged modelling was used to examine the interactive trajectories of change in parental psychopathology and mindful parenting during treatment. The results indicate that parent psychopathology significantly decreased and mindful parenting significantly increased from baseline to end-treatment. Lower levels of parent psychopathology at mid-treatment significantly predicted higher levels of overall mindful parenting when parents completed the PuP program (p = .006). Examination of the mindful parenting subconstructs revealed variation in the therapeutic process of change. Higher levels of non-judgemental acceptance of parent functioning at baseline predicted lower levels of parent psychopathology at mid-treatment (p = .02). Yet higher levels of emotional awareness of self and child at baseline predicted greater parent psychopathology at mid-treatment (p = .04). Lower levels of parent psychopathology at mid-treatment significantly predicted higher levels of compassion for self and child, non-judgemental acceptance of parent functioning, self-regulation in parenting, and listening with full attention upon completing the PuP program (ps < .05). Several key research, policy, and practice implications arose from this dissertation. First, policymakers and practitioners need to prioritise the implementation of evidence-based models in order to promote positive outcomes for families affected by substance misuse. Second, ongoing rigorous research is required to continue advancing the field, with particular attention to tracking longer-term outcomes and filling knowledge gaps. Third, families affected by parental substance misuse require a tailored approach depending on their unique presenting issues and may also require ongoing support beyond crisis points. Overall, the studies in this this dissertation underscore the powerful role parents play in the lives of children in families affected by parental substance misuse.