Informal settlements i.e., slums emerge from the interplay of multidimensional factors related to urbanization and sustainability. While the contribution of urban factors is well understood, the role of external drivers, such as uncontrolled migration to urban areas, is rarely addressed in research or policy-making. This study develops a novel conceptualization of slums by reviewing the pushing and pulling factors of migration and their contribution to informal settlements through 1) a socio-ecological system approach and 2) the concept of adaptive capacity. Further, it advances the discussion around synergistic and coherent policy-making in the urban context by reviewing three urban agendas and further using water as a case with the concept of cross-cutting domains. We show that the emergence of urban challenges can, and should be, linked to the root causes of flows into urban areas. Understanding these linkages through a socio-ecological system framework opens a window for knowledge-based policy development and addressing the question of how to avoid unsustainable urban development. Urbanization is one of the phenomena where the excessive complexity and dimensions of problems should not hamper action but instead, actions should be encouraged and enabled with synergistic and integrative pathways for sustainable urban development.