This special issue explores the realities of water provision in 'informal' urban spaces located in different parts of the world through eight empirical, case-based papers. The collection of articles shows that formality and informality are fluid concepts that say more about the authority to legitimate certain practices than describe the condition of that particular practice. In this introductory article we provide a historical overview that links the academic discussion on informality to urban water supply practices. Subsequently, we propose the concepts of disaggregation and co-production to describe how informality works, and how ideas about (in)formality are mobilised to label particular practices and service modalities. Disaggregation reveals that a single service delivery mechanism may incorporate activities, varying according to the degree to which they are formal or informal. Co-production describes a process where hybrid service provision modalities are produced as a result of the articulation of socio-political, economic, biophysical and infrastructural drivers. The article concludes by identifying a series of research directions that emerged as a result of producing this special issue