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This article investigates why the European Union (EU)-NATO rela tions on countering hybrid threats have been characterised more by cooperation rather than by rivalry. By examining the onset and development of the EU-NATO functional overlap in this policy area, the article explains the factors driving the two organisations’ coop eration. Using an eclectic approach that integrates institutional overlap theory and considers both systemic factors and unit-level factors , this research reveals how cooperation has evolved. A key focus is the role of informal relations and the pivotal function of the European Centre of Excellence for Countering Hybrid Threats (Hybrid CoE) in facilitating this cooperation. This research identifies three key findings. First, the shift in the international security envir onment following Russia’s 2014 invasion of Crimea was instrumen tal in prompting both the EU and NATO to prioritise countering hybrid threats, leading to functional overlap. Second, this overlap fostered cooperation rather than conflict, as both organisations recognised their mutual reliance on one another’s resources. Third, Hybrid CoE played a critical role in managing and supporting the EU-NATO functional cooperation by increasing the frequency and depth of interactions between the two organisations and by enhancing informal relations.