The third module of the Training Course presents the European Cos4Cloud project within the context of which this training course is implemented. Specifically, an analysis is made of the concept of ‘’Open Science’’ as a philosophy and vehicle for democratizing and streamlining scientific research, focusing also on the place and role of Citizen Science (CS) in this effort. The European Cos4Cloud project is presented in relation to how it attempts to respond to the challenge of Open Science. The project’s vision is described, its objectives and main lines of action are listed and the importance of integrating the various Citizens Observatories and developing new relevant CS services in the European Open Science Cloud (EOSC) is highlighted. The project is also portrayed in relation to the partners and Citizen Observatories participating in its implementation and a brief presentation is made of the special role played in it by the Environmental Education Lab (EEL, NKUA). All the above are approached in Module 3 as answers to the following questions: What is the concept of ‘’Open Science’’? What is the relationship of Citizen Science with Open Science? What are the vision, objectives and lines of action of the European Cos4Cloud project? Who are the contributors of the Cos4Cloud project? Which Citizens Observatories are participating in the Cos4Cloud project? What are the fields of action and initiatives promoted by the Environmental Education Lab (NKUA) through the Cos4Cloud project?