Background: The impact of parental drug use on children is a major public health problem. However, opioid-dependent fathers have been largely ignored in parenting research., Objective: To implement and test the feasibility and acceptability of the Parents under Pressure programme (PuP4Dads) for opioid-dependent fathers and their families, and to determine whether or not a full-scale evaluation could be conducted., Design: A mixed-methods feasibility study., Setting: Two non-NHS family support services for parents who use drugs in Scotland., Participants: Fathers prescribed opioid substitution therapy ( n = 25), their partners ( n = 17) and children, as well as practitioners, supervisors, service managers and referrers., Intervention: A home-visiting programme, including an integrated theoretical framework, case formulation, collaborative goal-setting and modules designed to improve parenting, the caregiving environment and child welfare. The programme was delivered flexibly over 6 months by accredited practitioners., Main Outcome Measures: Feasibility progression criteria included the recruitment target ( n = 24 fathers), acceptability of PuP4Dads, father engagement in the study (including a minimum of 66% of fathers completing PuP and a minimum of 10 fathers completing baseline and post-treatment research interviews), engagement in qualitative interviews (including a minimum of 10 fathers and 90% practitioner uptake and 80% manager uptake), focus groups (with a minimum of 80% referrer uptake), adequate fidelity and no adverse events., Data Sources: The following researcher-administered validated questionnaires were used: the Brief Child Abuse Potential Inventory, the Parenting Sense of Competence Scale, the Difficulties in Emotion Regulation Scale, the Paternal Antenatal Attachment Scale, the Maternal Antenatal Attachment Scale, the Emotional Availability Scale, the Brief Infant Toddler Social and Emotional Assessment, the Strengths and Difficulties Questionnaire, the Conflict Tactics Scale, Treatment Outcome Profile and the EuroQol-5 Dimensions, five-level version. Other sources included parent-completed service use (an economic measure), social work child protection data, NHS opioid substitution therapy prescription data and practitioner-reported attendance data. We also conducted interviews with fathers ( n = 23), mothers ( n = 14), practitioners ( n = 8), supervisors ( n = 2) and service managers ( n = 7); conducted focus groups with referrers ( n = 28); and held an ‘expert event’ with stakeholders ( n = 39) ., Results: The PuP4Dads was successfully delivered within non-NHS settings and was considered acceptable and suitable for the study population. Referrals ( n = 44) resulted in 38 (86%) eligible fathers, of whom 25 (66%) fathers and 17 partners/mothers consented to participate. Most fathers reported no previous parenting support. A total of 248 sessions was delivered to the 20 fathers and 14 mothers who started the intervention. Fourteen fathers (and 10 mothers) completed ≥ 6 sessions and six fathers (and four mothers) completed ≤ 5 sessions. Father and mother attendance rates were equal (mean 71%). Median length of engagement for fathers was 26 weeks and for mothers it was 30 weeks. Twenty-three fathers completed interviews at baseline, 16 fathers completed interviews at follow-up 1 and 13 fathers completed interviews at follow-up 2. Outcome measures were well tolerated; however, the suitability of some measures was dependent on family circumstances. The researcher-administered questionnaires had few missing data. The perceived benefits of PuP4Dads reported by parents, practitioners and managers included the following: the therapeutic focus on fathers improved parental emotion regulation, there was improved understanding and responding to child’s needs, there was better multiagency working and the programme was a good fit with practice ‘ethos’ and policy agenda. Learning highlighted the importance of service-wide adoption and implementation support, strategies to improve recruitment and retention of fathers, managing complex needs of both parents concurrently, understanding contextual factors affecting programme delivery and variables affecting intervention engagement and outcomes., Limitations: Lack of emotional availability and economic (service use) data., Conclusions: A larger evaluation of PuP4Dads is feasible., Future Work: Further work is required to demonstrate the effectiveness of PuP4Dads and the cost implications. A better understanding is needed of how the intervention works, for whom, under what circumstances and why., Trial Registration: Current Controlled Trials ISRCTN43209618., Funding: This project was funded by the National Institute for Health Research (NIHR) Public Health Research programme and will be published in full in Public Health Research ; Vol. 10, No. 3. See the NIHR Journals Library website for further project information., (Copyright © 2022 Whittaker et al. This work was produced by Whittaker et al. under the terms of a commissioning contract issued by the Secretary of State for Health and Social Care. This is an Open Access publication distributed under the terms of the Creative Commons Attribution CC BY 4.0 licence, which permits unrestricted use, distribution, reproduction and adaption in any medium and for any purpose provided that it is properly attributed. See: https://creativecommons.org/licenses/by/4.0/. For attribution the title, original author(s), the publication source – NIHR Journals Library, and the DOI of the publication must be cited.)