Authorship is more than a name on a publication: it signifies intellectual contribution, responsibility, and accountability. It encompasses the creation of ideas, the writing process, and the ethical principles that ensure transparency and fairness in collaborative research. Beyond recognition, authorship carries legal implications through intellectual property rights (IPR), granting authors control over the reproduction, distribution, and use of their work. It also serves as a safeguard against plagiarism and unauthorized use.Recurring challenges observed across multiple EU-funded projects have highlighted the need for clear, practical guidance on authorship. This document responds to that need. Drawing on established frameworks such as the Horizon Europe Programme, the Annotated Grant Agreement, and the European Code of Conduct for Research Integrity, it consolidates key legal provisions, ethical standards, and best practices. Its purpose is to help research teams: Understand the legal and ethical dimensions of authorship; Apply consistent criteria for identifying authors and contributors; Prevent disputes through transparent agreements; Ensure fair recognition of all contributions. While this guideline does not aim to be exhaustive, it provides a practical reference for navigating authorship in interdisciplinary and international collaborations. By promoting clarity and integrity, it supports the values of responsible research and strengthens trust among partners.