Scientific evidence about the negative health effects of urban environmental exposures is mounting. Yet key scientific gaps exist. Surveys show that citizens are increasingly concerned about the consequences of these exposures on their own health, and are engaged in data collection and activism efforts around problems such as urban mobility and air and noise pollution. These concerns, along with the availability of affordable crowd-sensing and data processing technologies that allow citizens to measure environmental and health parameters, make environmental epidemiology studies an ideal, yet underexplored opportunity to develop citizen science projects. Enabling collaboration between researchers and citizens to generate solid, unbiased scientific evidence of local relevance can reduce existing information gaps. It can empower people to contribute to novel and bottom-up research agendas, interventions and co-creation of public policies. The aim of the Citizen Science Project on Urban Environment and Health (CitieS-Health) is to develop an effective citizen science model at the maximum collaboration level. The project will develop citizen science projects in five diverse European cities (Barcelona, Kaunas, Ljubljana, Amsterdam, Lucca), assessing urban air and noise pollution, wood burning, urban design and mobility at local levels. An innovative aspect of CitieS-Health is studying the link between these exposures and health impacts. Citizens will participate in defining research questions, designing and implementing studies, and analysing, interpreting and communicating results. The projects will inform the first open toolkit for the development and promotion of citizen science projects in urban environment and health. The project will also co-design a set of governance principles and procedures to allow participants control over project data and outcomes, and will contribute indicators to assess the project's impacts on different sectors.