This thesis explores the role of the informal economy in urban environmental management. Cities' relation with the environment is mediated by the urban infrastructure, which provides services such as transport or waste management. Beyond the implementation of plans by local governments, the daily operation of such infrastructure is also the result of informal work. Yet, little is known about the nature and impacts of informal work in urban service provision. This thesis tackles this research gap by documenting the everyday operation of domestic waste collection and management in a neighbourhood of Mexico City. The aim of this research is twofold. Firstly, it aims to critically analyse the concept of 'informality' in the case of urban waste management, and to document how informality operates in that context. Secondly, it assesses the contribution of the informal economy to the waste management system, by contrasting it to the key components of urban sustainability. The research presents primary data collected through qualitative fieldwork. Using an urban metabolism framework, it documents waste flows through the urban infrastructure, identifying the role of formal and informal waste handlers along the way. In parallel, it explores the normative discourses of informality that are mobilised in the production of Mexico City's urban sustainability policies. The thesis argues that it is necessary to re-consider the role of informal workers in urban sustainability. In Mexico City, informal and formal waste workers' relationship is symbiotic. Formal waste collection services are sustained by informal work and cash flows. In parallel, informal waste handlers provide the main input (recyclable materials) to the formal recycling industry - this is achieved through the reliance on local solidarity networks and techniques of experimentation and innovation which are characteristic of the informal economy. The informal economy appears to contribute positively to the environmental and social components of urban sustainability. Yet, informal workers are not recognised as legitimate actors in policy making. Instead, the concept of informality is mobilised by civil servants to exclude informal workers from the policy process. This challenges the potential for inclusive governance, a key component of urban sustainability.