This book aims to identify ways of overcoming the limitations of the communicative tradition in understanding participatory spatial planning. Three conceptual models that offer different perspectives on public and civic participation in complex urban planning processes are presented and reviewed: the consensual model, which conceives of planning as a collective decision-making practice geared toward consensus building and conflict resolution; the conflictual model, which views planning as a social mobilization practice addressed at empowerment of marginalized groups; and the trading zone model, which reframes collaborative planning as a coordination activity with respect to practical proposals in the presence of unstable and conflicting rationalities and values. The controversial story of the Integrated Intervention Program “PII Isola Lunetta” in Milan is examined through the interpretative lenses of these models, with detailed interpretation of how each model performs in the field. The book concludes by offering critical reflections on the reframing of participatory spatial planning, highlighting the value of trading zones/trading languages and boundary objects as tools for understanding and addressing collaborative practices in complex and conflictual urban planning processes.