This working paper presents the findings of a study of Co-operative Compliance in the Netherlands and the UK. These two countries were early adopters of Co-operative Compliance as a mechanism for managing the relationships between large business taxpayers and the tax authorities, mediated to various degrees by tax advisers. The juxtaposition of these two cases provides interesting insights into how policy initiatives come into being and evolve, as well as how regulators learn from each other in subtle, and not so subtle ways. Policy learning in this context is promoted by the intervention of the OECD as promulgator of best practices in tax administration. Our focus is on Co-Operative Compliance in these two jurisdictions in practice. We examine how highly skilled actors perceive the programme in retrospect and prospect, by reference to their lived experiences derived from interviews. We also chart the emergence and subsequent adaptations of the programmes through the lens of official pronouncements and policy documents. The project started in 2013 and is generously funded by Horizon 2020. Over the course of the project, the objects of study were constantly moving within and between the countries we study, as was the backdrop of global events and developments in other jurisdictions. Capturing the essence of such a dynamic environment has been challenging but rewarding. Sections 2 and 3 of this report provides a brief background to the project and an explanation of our methodology respectively. This is followed by descriptions of the working practices in both jurisdictions in Sections 4 and 5. Section 6 is the heart of the report that builds on the background provided in earlier Sections and presents the views of large businesses of various dimensions of Co-operative Compliance in both countries. In Section 7 we offer a discussion of our findings together with our conclusions.