This publication presents the challenges and opportunities confronting early childhood development in vulnerable urban contexts, derived from specialised research by Arup and the Bernard van Leer Foundation (BvLF). The data is clear: vulnerable urban areas such as refugee and informal settlements house a growing population in critical need, and the number and size of these areas will only increase in the coming decades. Living in these contexts has particularly significant negative impacts on young children aged 0 to 5. At present, governments, development and humanitarian organisations, and urban practitioners devote little attention to the specific needs of the 0-5 age group in projects aimed at improving conditions in informal and refugee settlements. This age group's needs are different than those of older children but are often 'lumped in' with them from a planning and policy perspective, or worse, go entirely unrecognised. In addition, the complexity of vulnerability in these contexts makes it difficult to design and implement effective early childhood development solutions that consider the influence of the built environment. Arup and BvLF have partnered to help bridge this gap. The Proximity of Care approach was developed to better understand the needs and constraints faced by young children, their caregivers, and pregnant women in informal and refugee settlements; and to ultimately help improve their living conditions and well-being. [This report was co-produced by Arup.]