The topic of the Transnational Collective Agreements (TCAs) is one of the most challanging issues for labour law because of its international dimension, the absence of a legal framework of reference, both at the national and supranational (regional and international) level. Its appearence 30 years ago gave rise to a movement of transnational agreements that has strenghtened especially in the years 2000, until reaching the significant amount of more than 300 agreements concerning about 10 million workers worldwide. These are experimental forms of regulation, of a private nature but which for some traits rest on public tools. The growing hybridization between collective private and public regulatory forms, which are source of inspiration, institutional support mechanisms, base contents of the agreements, support to effectiveness, is one of the main features of the current development of the TCAs. The contamination between tools from different sources, mostly soft law, and their adaptation to specific regulatory needs of individual agreements, appears to be the result of settling experiences, learning processes and searching for more reliable mechanisms to ensure effectiveness as well as growing attention to the socially responsible behaviour of companies operating on a global scale, through the improvement of the voluntary mechanisms of CSR and provision of dispute settlement mechanisms that tend to raise the rate of compliance.