There have been significant research studies on informal settlements within various disciplines including sociology, economy, politics, governance, and urbanism. However, little is known about the complexity and dynamism of informal settlements. The purpose of this study is to develop a framework for understanding the multiplicity of factors influencing the formation and transformation of informal settlements. It examines and validates various intricacies characterising informal settlements in three ways. First, informal settlement characteristics and their relationships are explored. Second, growth and transformation variables are examined. Third, qualities of the informal urban form and those that relate to sustainability are juxtaposed. Utilising two case studies from Greater Cairo, a qualitative approach is adopted including a critical analysis of the literature, interviews with experts and academics, and field observations. Through a comprehensive investigation of informal settlements, two deductions were made: First, the critical physical, social, and economic characteristics that influence their growth were identified. Second, the unique correlations between these characteristics were established and verified by the two case studies. The correlations assist in establishing the logic and dynamics of the informal settlements that can then be applied to develop intervention strategies. In addition, the inferred informal urban form can be considered as a sustainable urban form tailored for further analyses of informal settlements of cities of the global south.