In this dissertation, I argue that a theory of colonial internationalism is necessary, or even indispensable, to adequately understand and explain the origins and the endurance over time of colonialism. International cooperation among colonial experts reached its climax in the foundation of the International Colonial Institute (ICI, 1893-1982) whose membership reached 200 in 1914. This non-governmental institute was the most important international and colonial institution prior to the First World War. It developed into a hub of exchange between colonial experts, who contributed in a significant way to making colonial domination more efficient and to establishing a form of best practice of colonial rule through comparison and knowledge transfers. In the interwar period, the ICI provided the League of Nations' Permanent Mandates Commission (PMC) and the International Labor Organization (ILO) with colonial experts. Taking the ICI as a starting point, my dissertation explores the international dimension of colonialism between 1830 and 1950. I investigate a broad range of colonial methods that internationalist experts designed, tested and applied in the colonies. Their impact could be felt across the colonial world. Influenced by a racist notion of tropical hygiene, they dismissed settler colonization and proclaimed the "triumph of the native races" as potential co-colonizers. To train specialists on colonized populations, they professionalized the training of colonial administrators in all colonizing countries and founded new schools for overseas administrators. Members of the ICI invented legal anthropology as a means to manipulate customary law, while others modified Islamic law to use it for colonial purposes. International cooperation among colonizers was also responsive to Pan-Islamic movements across colonial empires, but colonial administrations ultimately learned to use Pan-Islamism for their own purposes. With regard to colonial economies, they established a professionalized (but not necessarily successful) cash crop production by transferring successfully tested seeds and plants from agronomic laboratories in the Dutch Indies to Africa. The main argument of my dissertation is that international transfers among colonial experts brought about development policies and a certain degree of cooperation with the indigenous populations. Far from granting the colonized a say, however, the colonizers attempted to profit from their collaboration without treating them on equal terms. While modernizing and professionalizing colonial domination and exploitation, colonial internationalists also legitimized and sustained colonial domination. After 1945, the ICI contributed to applying colonial patterns of thinking to the emerging "Third World." Given this longue durée success of colonial internationalism, this dissertation calls for an internationalist theory of colonialism.