Smallholder farmers in developing economies are key suppliers in agri-food value chains, yet often lack capabilities to meet quality, reliability, and sustainability expectations. This paper presents a systematic literature review of empirical studies on farmer development, conceptualised as supplier development at the farm gate to examine who builds farmers’ capabilities, which initiatives are implemented, and with what sustainability outcomes. Searches of a multidisciplinary library discovery service and Scopus identified 15 studies reporting implemented farmer development initiatives and farmer-level outcomes in developing economies. The synthesis shows that capability building is dominated by government and non-governmental organisation-led programmes, typically bundling training, extension, input provision, and financial support, while buyer-led initiatives are rare and performance is measured mainly in economic terms, with social and environmental dimensions under-specified. The review positions smallholder capability building within supply chain management and argues that building smallholder capabilities is both a development imperative and a strategic supply chain task.