The Deliverable 2.1, Case Studies System Requirements and Functional Specification, documents and reports on the collaborative definition and formalisation of the identified user needs, requirements and specification of functionalities for the Early Warning System and digital tools for the PLANET4HEALTH Case Studies, having been performed during the first year of the Project. It presents the approach and key steps of the applied methodology, as well as the contents, interactive feedback and outcomes of the co-creation sessions and workshops with the stakeholders, formulated usage goals and scenarios, and derived functional and non-functional requirements systematisation and specification for the tools (explained and described in Chapter 5). The collaborative work undertaken during the internal joint definition sessions has involved all 17 PLANET4HEALTH consortium partners, and the subsequent consultation and co-creation workshops also involved external stakeholders and actors. These sessions and workshops have played a crucial role in formulating the objectives and functionalities of the EWSs and digital tools, ensuring they meet the diverse needs and requirements of the end users and other involved stakeholders on the four PLANET4HEALTH Case Studies, including the initial identification, systematisation and segmentation of these stakeholders per types and roles, and degree of involvement and influence on the development and usage. The overview of identified key constraints and barriers affecting the requirements and specified features, as well as preliminary insights into the available datasets, foundations for the EWSs and tools, have also been provided, concluding with the overview of next planned steps. These comprehensive collaborative and co-design efforts pave the way as crucial inputs driving the upcoming development of the PLANET4HEALTH EWSs and digital interactive tools in the parallel and continuing Task 2.3 (Development of the Early Warning System and Digital Tools for CSs).