The close interactions between humans, animals, and the environment, can lead to the emergence of infectious diseases through spillover events. Early warning surveillance systems at the human-animal-environment interface are pivotal to enable timely public health actions. Work Package 4 (WP4) of UNITED4Surveillance aims to establish and strengthen partnerships in a One Health approach, emphasizing structured data sharing for effective public health surveillance. This is achieved by supporting European Union Member States and Join Action partner countries in developing One Health surveillance structures at the national level that integrate data from human, animal, and environmental domains. WP4 is organized around three disease groups: foodborne diseases, zoonotic influenza, and vector-borne diseases. The tasks within each group encompassed three subtasks: 1) goal definition and stakeholder analysis, 2) systems mapping of the current and desired situation, and 3) piloting promising approaches. The first two subtasks were executed collaboratively across disease groups to ensure a consistent methodology. Countries aimed to align on specific pathogens within each disease group for shared experiences. The results of these tasks are described in a previous deliverable (D4.1). Based on the first two tasks, piloting of implementation of One Health surveillance systems (task 3) were conducted. Pilots were designed at country-level to consider the different pace One Health is developed in the countries, as well as locally relevant needs. One of the major lessons from the pilots were the importance of early, open, and structured communication among all relevant sectors, such as public health, animal health, environmental health, laboratories, and policymakers. Establishing trust and maintaining regular information exchange are crucial for effective cooperation and knowledge sharing. Early engagement of stakeholders, including public institutions, clinical laboratories, commercial partners, and the public, is essential to secure support and active participation, especially in the absence of regulatory requirements. Transparency about goals and data use, as well as clear agreements about data management, further strengthen collaboration and trust. Flexibility has also proven vital, as many pilots encountered practical or legal obstacles that required quick adaptation and ongoing evaluation of strategies. The integration of modern, user-friendly IT platforms and technologies such as whole genome sequencing (WGS) has significantly improved data sharing and surveillance capabilities. Open-source solutions and continuous training help address resistance to centralized data integration. Furthermore, efficient and timely laboratory procedures, along with the implementation of advanced diagnostic methods, are fundamental to effective surveillance. Although U4S has made considerable progress, maintaining these improvements over time will depend on securing stable funding, continued political backing, and lasting engagement from all stakeholders. The project’s outcomes highlight the importance of embedding One Health surveillance into standard public health systems, which is particularly vital given the growing challenges posed by climate change and emerging infectious diseases.