Since the early 1990s, World Bank officials in many countries have pressed their government borrowers to include nongovernmental organizations as development partners. What impact has this new partnership norm had in the bank's borrower countries, and why? This article investigates these questions through longitudinal analysis of three cases: Guatemala, Ecuador, and the Gambia. In their first iteration in the 1990s, these bank-sponsored efforts generally failed to take root; yet by the 2000s, NGOs and state actors were engaged in multiple partnerships. This article suggests that over time, bank officials' repeated efforts to embed these new ideas fostered a social learning process that led NGOs to adopt more strategic partnership practices and government officials to see NGO partners as useful. Several factors may affect this learning process: levels of professionalism and the growth of professional networks, the presence of effective "bridge builders," and the level of historical conflicts.