Background: Worldwide, women fail to reach the recommended exclusive breastfeeding target of 6 months postpartum. The objective of this study was to present a conceptual and methodological synthesis of interventions designed to promote exclusive breastfeeding to 6 months in high-income countries. Materials and Methods: A systematic search of leading databases was conducted for scholarly peer-reviewed randomized trials published between January 2000 and June 2013. Seventeen articles were identified as relevant; all were published in English and assessed exclusive breastfeeding with a follow-up period extending beyond 4 months postpartum. Articles were analyzed for overall quality of evidence, regarding duration of exclusive breastfeeding, using the Grading and Recommendations Assessment, Development, and Evaluation approach. Results: A significant increase in the duration of exclusive breastfeeding was found in eight of the 17 studies, with most interventions using supportive or educational approaches. Interventions in pregnancy focused on educating mothers on the benefits of exclusive breastfeeding. Fifteen interventions took place, at least in part, in the postnatal period and provided educational and emotional support to mothers. Of the eight successful interventions, five took part in the postnatal period in the mothers' own homes. The quality of the evidence for duration of exclusive breastfeeding was moderate. Conclusions: The most successful interventions were conducted in the postnatal period and over a long period of time; however, the findings were inconsistent. No study assessed intervention fidelity, and only two studies noted maternal body mass index, a variable known to affect exclusive breastfeeding rates. Further research is needed to provide a robust evidence base to inform future interventions.