William III, the main opponent of William III, surrounded himself with a small circle of confidants who played an important role in advising him. The most prominent among these was undoubtedly Hans Willem Bentinck, 1st Earl of Portland (1649-1709). This thesis analyses Portland's career within the context of three historiographical debates. The first is the tendency of British historians now to connect the histories of the three British Kingdoms, within an international context. The second is the recent interest in the phenomenon of the favourite as a pivotal figure in 17th century state-building. The third is the argument of British revisionist historians to explain the rise of Britain as a great power from the fundamental changes that swept the British Isles after 1688. The thesis consists of 7 chapters, grouped into three parts. The first part deals with Portland's early career. Chapter 1 deals with the formative years until 1688. The next chapter is a case study, analysing his political, military and diplomatic activities at the eve of the Glorious Revolution. The second part forms the core of the thesis and deals with the period between 1688 and 1697, the climax of his career as an Anglo-Dutch favourite. Chapter 3 analyses Portland's influence at court, the army, the diplomatic service and the Parliament. It also distinguishes his influence in Scotland, Ireland, England and the Dutch Republic. The next chapter deals with Portland's political and military activities during the Nine Years War. Chapter 5 discusses his role in the development of a specific Williamite ideology, and popular and political opposition and criticism of the favourite. The third and last part concentrates on Portland's last years. Chapter 6 looks at the causes for his retirement from active politics in 1699. The last chapter describes his diplomatic activities during the last decade of his life. Conclusions have been drawn on two levels. Firstly, it seems that the estimation of Portland's political role in historiography has been erroneous. Portland was neither a weak favourite to a strong king, nor an almighty favourite. In fact, he started his career in England as a military 'secretary', but increasingly gained influence in other spheres as well, such as parliamentary and financial management. A more general conclusion can be made. Portland's increasing influence in several core spheres of Williamite government forms the key to understanding his role as Anglo-Dutch favourite. The rise of the fiscal-military state as a result of the Nine Years War was a pivotal development. The war required strong government, and in turn led to the rise of Parliament. Portland played an important role in both the army and Parliament. Moreover, the Dual Monarchy of the United Provinces and the British Isles under the King-Stadholder, effectively a composite monarchy at war, required co-ordination, in which Portland played a vital role. Understandably, if the rise of the Anglo-Dutch favourite can be related to the Nine Years War and the Dual Monarchy, the loss of his power was the result of the end of the war and the desintegration of the personal union under William during the late 1690s. The Anglo-Dutch favourite, therefore, was a unique figure related to the specific circumstances that swept the British Isles after 1688, the Nine Years War and the Anglo-Dutch union.