Summary: Cidade de Deus serves as the inspiration and case study for 'Cidade de Deus - City of God' - a widely relevant study of the appropriation and customization of standardized mass housing over time. The once-notorious favela in Rio de Janeiro, which grew out of a government-housing program, is today a safe and vibrant neighborhood and a viable counter model to gated condominiums. This research-based design study by the Master of Advanced Studies in Urban Design program at the ETH Zurich unpacks the implications of Cidade de Deus' hybrid environment, as it exists today. What results is a model for improving mass housing programs using the ingenuity of informal practices - a model applicable not just in Brazil, but world-wide.