Most of the world’s poor live in slums. As a result, for more than half a century, international donors and national governments have grappled with the challenges of providing them with housing and infrastructure in a sustainable way. Though considerable advances have been made, the implementation problems are still disappointingly high. I chose to research an internationally-recognized model in upgrading thousands of squatter households, in order to understand the secrets of its success. Started in 1993 in Salvador de Bahia, Brazil, the Ribeira Azul slum upgrading program has provided housing and basic infrastructure to 3,500 households living on stilts, and improved infrastructure for another 150,000 inhabitants. My inquiry, surprisingly, revealed two unexpected findings. First, some of the principal features said to explain the model’s success proved to be considerably weaker than claimed. Second, Ribeira Azul proved to be successful in certain ways that have been ignored in its categorization as a “model.” This study highlights practical lessons for project design and implementation, and can be used by professionals in the project evaluation and urban development fields.