BACKGROUND: Parental substance use is a substantial public health and safeguarding concern. There have been a number of trials of interventions relating to substance-using parents that have sought to address this risk factor, with potential outcomes for parent and child. OBJECTIVES: To assess the effectiveness of psychosocial interventions in reducing parental substance use (alcohol and/or illicit drugs, excluding tobacco). SEARCH METHODS: We searched the following databases from their inception to July 2020: the Cochrane Drugs and Alcohol Group Specialised Register; CENTRAL; MEDLINE; Embase; PsycINFO; CINAHL; Applied Social Science (ASSIA); Sociological Abstracts; Social Science Citation Index (SSCI), Scopus, ClinicalTrials.gov, WHO ICTRP, and TRoPHI. We also searched key journals and the reference lists of included papers and contacted authors publishing in the field. SELECTION CRITERIA: We included data from trials of complex psychosocial interventions targeting substance use in parents of children under the age of 21 years. Studies were only included if they had a minimum follow-up period of six months from the start of the intervention and compared psychosocial interventions to comparison conditions. The primary outcome of this review was a reduction in the frequency of parental substance use. DATA COLLECTION AND ANALYSIS: We used standard methodological procedures expected by Cochrane. MAIN RESULTS: We included 22 unique studies with a total of 2274 participants (mean age of parents ranged from 26.3 to 40.9 years), examining 24 experimental interventions. The majority of studies intervened with mothers only (n = 16; 73%). Heroin, cocaine, and alcohol were the most commonly reported substances used by participants. The interventions targeted either parenting only (n = 13; 59%); drug and alcohol use only (n = 5; 23%); or integrated interventions which addressed both (n = 6; 27%). Half of the studies (n = 11; 50%) compared the experimental intervention to usual treatment. Other comparison groups were minimal intervention, attention controls, and alternative intervention. Eight of the included studies reported data relating to our primary outcome at 6- and/or 12-month follow-up and were included in a meta-analysis. We investigated intervention effectiveness separately for alcohol and drugs. Studies were found to be mostly at low or unclear risk for all 'Risk of bias' domains except blinding of participants and personnel and outcome assessment.  We found moderate-quality evidence that psychosocial interventions are probably more effective at reducing the frequency of parental alcohol misuse than comparison conditions at 6-month (mean difference (MD) -0.32, 95% confidence interval (CI) -0.51 to -0.13; 6 studies, 475 participants) and 12-month follow-up (standardised mean difference (SMD) -0.25, 95% CI -0.47 to -0.03; 4 studies, 366 participants). We found a significant reduction in frequency of use at 12 months only (SMD -0.21, 95% CI -0.41 to -0.01; 6 studies, 514 participants, moderate-quality evidence). We examined the effect of the intervention type. We found low-quality evidence that psychosocial interventions targeting substance use only may not reduce the frequency of alcohol (6 months: SMD -0.35, 95% CI -0.86 to 0.16; 2 studies, 89 participants and 12 months: SMD -0.09, 95% CI -0.86 to 0.61; 1 study, 34 participants) or drug use (6 months: SMD 0.01, 95% CI -0.42 to 0.44; 2 studies; 87 participants and 12 months: SMD -0.08, 95% CI -0.81 to 0.65; 1 study, 32 participants). A parenting intervention only, without an adjunctive substance use component, may not reduce frequency of alcohol misuse (6 months: SMD -0.21, 95% CI -0.46 to 0.04, 3 studies; 273 participants, low-quality evidence and 12 months: SMD -0.11, 95% CI -0.64 to 0.41; 2 studies; 219 participants, very low-quality evidence) or frequency of drug use  (6 months: SMD 0.10, 95% CI -0.11 to 0.30; 4 studies; 407 participants, moderate-quality evidence and 12 months: SMD -0.13, 95% CI -0.52 to 0.26; 3 studies; 351 participants, very low-quality evidence). Parents receiving integrated interventions which combined both parenting- and substance use-targeted components may reduce alcohol misuse with a small effect size (6 months: SMD -0.56, 95% CI -0.96 to -0.16 and 12 months: SMD -0.42, 95% CI -0.82 to -0.03; 2 studies, 113 participants) and drug use (6 months: SMD -0.39, 95% CI -0.75 to -0.03 and 12 months: SMD -0.43, 95% CI -0.80 to -0.07; 2 studies, 131 participants). However, this evidence was of low quality. Psychosocial interventions in which the child was present in the sessions were not effective in reducing the frequency of parental alcohol or drug use, whilst interventions that did not involve children in any of the sessions were found to reduce frequency of alcohol misuse (6 months: SMD -0.47, 95% CI -0.76 to -0.18; 3 studies, 202 participants and 12 months: SMD -0.34, 95% CI -0.69 to 0.00; 2 studies, 147 participants) and drug use at 12-month follow-up (SMD -0.34, 95% CI -0.69 to 0.01; 2 studies, 141 participants). The quality of this evidence was low. Interventions appeared to be more often beneficial for fathers than for mothers. We found low- to very low-quality evidence of a reduction in frequency of alcohol misuse for mothers at six months only (SMD -0.27, 95% CI -0.50 to -0.04; 4 studies, 328 participants), whilst in fathers there was a reduction in frequency of alcohol misuse (6 months: SMD -0.43, 95% CI -0.78 to -0.09; 2 studies, 147 participants and 12 months: SMD -0.34, 95% CI -0.69 to 0.00; 2 studies, 147 participants) and drug use (6 months: SMD -0.31, 95% CI -0.66 to 0.04; 2 studies, 141 participants and 12 months: SMD -0.34, 95% CI -0.69 to 0.01; 2 studies, 141 participants). AUTHORS' CONCLUSIONS: We found moderate-quality evidence that psychosocial interventions probably reduce the frequency at which parents use alcohol and drugs. Integrated psychosocial interventions which combine parenting skills interventions with a substance use component may show the most promise. Whilst it appears that mothers may benefit less than fathers from intervention, caution is advised in the interpretation of this evidence, as the interventions provided to mothers alone typically did not address their substance use and other related needs. We found low-quality evidence from few studies that interventions involving children are not beneficial.