Globally, states use rural-to-urban resettlement to fuel development, yet this formal process consistently generates its own informalities. Using a comparative case study of China—contrasting its affluent coast with its poorer hinterland—this book reveals how informality not only persists after resettlement but performs essential functions, critically challenging the effectiveness of prevailing policies. Theoretically, the study leverages the innovative Credibility Thesis, applying its Formal, Actual, and Targeted (FAT) Institutional Framework and Credibility Scales and Intervention (CSI) Checklist to explain the emergence and evolution of post-resettlement informality. The findings offer powerful, empirically grounded recommendations for integrating informal realities into urban planning, with profound implications for understanding institutional credibility and the functional role of informality in development.