NAUTILOS, a Horizon 2020 Innovation Action project funded under EU’s the Future of Seas and Oceans Flagship Initiative, aims to fill in marine observation and modelling gaps for biogeochemical, biological and deep ocean physics essential ocean variables and micro-/nano-plastics, by developing a new generation of cost-effective sensors and samplers, their integration within observing platforms and deployment in large-scale demonstrations in European seas. The principles underlying NAUTILOS will be those of the development, integration, validation and demonstration of new cutting-edge technologies with regards to sensors, interoperability and embedding skills. The development will always be guided by the objectives of scalability, modularity, cost-effectiveness, and open-source availability of software products produced. Bringing together 21 entities from 11 European countries with multidisciplinary expertise, NAUTILOS has the fundamental aim to complement and expand current European observation tools and services, to obtain a collection of data at a much higher spatial resolution, temporal regularity and length than currently available at the European scale, and to further enable and democratize the monitoring of the marine environment to both traditional and non-traditional data users.