Urban Living Labs (ULLs) have been implemented in many cities, but their organizational and legal structure has not often been analyzed. ULLs aim to provide a space for different parties to research, develop, and test solutions to urban problems whilst engaging with local communities. Their experimental approach to urban innovation and to public-private collaboration makes flexibility, openness, and informality important. However, ULLs are also confronted with existing legal frameworks, responsibilities, and liabilities. Whilst they aim at shared decision-making and horizontality, they must navigate public and private interests, and interact with local government as well. To understand these dynamics, this article examines the legal and organizational structures of ULLs, the factors and trade-offs that influence it, and the role municipal government plays in in these structures. This article analyzes the different forms and trajectories of ULLs in practice, through semi-structured interviews held in four labs in Amsterdam. Through qualitative research, we found that 1) ULLs are partnerships that exist on a spectrum of formalization, from informal to highly formal; 2) the degree of formalization is influenced by financial, legal, and organizational factors that change over time; 3) each degree of formalization is associated to trade-offs, even if these trade-offs are not explicitly formulated by the people involved; 4) tensions arise from the municipality's double role as public authority and as partner. We conclude that ULLs could gain from clearly identifying the legal frameworks that condition their structure, actions, and future.