CitiObs will consolidate and apply tools and practice-based knowledge for co-creating data, knowledge and local action via Citizen Observatories (COs): these tools will enhance existing and new citizen observatories to engage citizens and marginalised communities, add value to environmental observations in the urban context, increase and validate citizen observations of the urban environment as part of the existing in-situ Earth Observation systems, co-create inclusive local actions for sustainability and ensure that CO data contributes to research and policy development towards the objectives of the European Green Deal. To ensure broad use, the CitiObs tools and approaches will be developed in co-creation with COs in 5 Frontrunner cities, finetuned with 30 Implementer cities and showcased to 50 Fellow cities. The pressure on cities to speed up the process of emissions reduction from pollutants and climate gases is overwhelming. The required measures are likely to be found not only in technological development but also in the required social transformations. Theseis transformations includes increasing citizen involvement and engagement with science and policy, increasing awareness of the underlying issues and promoting societal actions to protect the urban environment. Technological innovations need to be considered within the context of multi-level vertical and horizontal governance structures where citizen participation is recognized and included. CitiObs will support citizen observatories in distinct cities to create/enhance/or scale up inclusive and diverse citizen observatories, fostering in particular an active role of citizens in the observation of the urban environment using low-cost sensor technologies and wearables, with a particular focus on air quality and related environmental measures. CitiObs will formalise, valorise and legitimise the role of citizen observations.