WeSenseIt is a project about citizen involvement in environmental monitoring with particular focus on water, water-related variables, and the management of flood risks. We define a Citizen Observatory in a wide sense, where actors can actively and/or passively contribute to observation, data acquisition, and information transmission. There are different levels of citizen engagement, expertise, and a priori knowledge, and a wide range of technologies from simple to complex based both on physical and crowd-sourced approaches. In this project, we explore the ensemble of these technologies and develop specific tools suitable for a citizen observatory of water and even beyond. Proposed solutions include novel sensor types for the measurement, recording or documentation of water-related variables, combinations of simple hardware and sensing infrastructure with software tools and applications, and information transmission such as observations, pictures, videos or reports using smartphones and social media channels. An additional objective is also to significantly reduce cost of sensors and monitoring systems permitting an increased number of deployed sensors for better coverage and representation of spatial variability. Another important aspect considered and developed in this project is power consumption optimization of sensing systems and their autonomy to allow for a maximum of flexibility in the use and deployment such sensors. Lastly, an emphasis is on mobile sensing technology, which is expected to close the gaps in most static monitoring networks. This is where citizens become the main actors and where the potential of contributing to the observatory is largest and of biggest impact. The WeSenseIt technologies are presented based on the results from the project’s case studies.