Water services regulation has been introduced to the Kenyan water sector as part of a comprehensive sector reform. The research reported in this paper analyses institutional arrangements and the legal framework for formal water service provision and regulation in Nairobi as well as the informal and largely unregulated water markets serving the majority of the population living in the city's many informal settlements. By tracing regulatory developments against a backdrop of steadily increasing population and high poverty levels, the paper examines the role of regulation in facilitating the early delivery of an appropriate level of service to low-income households. By 2006, regulation had only begun to address the significant gaps and ambiguities in the available baseline data and corresponding sector targets, and to include low-income consumers and their current providers into the regulatory framework, while a lack of financial viability prevented the main provider from expanding services into underserved areas. The findings highlight a continued need for institutional strengthening and good regulatory governance to drive pro-poor sector development.