Emotional and behavioural problems frequently begin in early childhood. With fathers becoming increasingly involved in young children’s care there is a need to understand the impact paternal parenting has on children’s early development and to involve fathers in parenting interventions that so frequently leave fathers out. Part one of this thesis is a meta-analytic review examining whether paternal parenting is reliably associated with internalizing problems in young children. Part two of this thesis is a qualitative investigation into fathers’ experiences of and engagement in an attachment-based parenting programme (Video-feedback Intervention to Promote Positive Parenting and Sensitive Discipline; VIPP-SD). Part two also sought to elucidate mothers’ perspectives of fathers’ involvement in VIPP-SD. Part two of this thesis formed a qualitative sub-study of a larger multi-site randomized controlled trial ‘Healthy Start, Happy Start’ (HS, HS). Part three is a critical appraisal of the research process which primarily considers the impact my dual role as clinician and researcher in HS, HS had on the research process.