Background: Existing evidence suggests that when standalone breastfeeding support interventions are offered to women who choose to breastfeed, the duration and exclusivity of breastfeeding are likely to be increased (Gavine et al., 2022). However, many women in the UK and elsewhere continue to report that lack of adequate breastfeeding support results in them stopping breastfeeding earlier than planned. Therefore, one key research question is now to identify how known effective interventions can successfully be implemented in practice. As part of the of the Action4Breastfeeding study, this review aimed to synthesise existing evidence on factors affecting the implementation of effective interventions identified in the updated Cochrane review...