Migration has been an integral part of human activity for as long as people have inhabited the earth. Whether moving as individuals, families or tribes, migration is an age-old response to the physical need for food, shelter and security, and the psychological need for adventure and exploration. However, while migration has marked all periods of human history, the phenomenon of ‘international migration’ had to await the reordering of the geopolitical landscape as a collection of territorial States in which governments had authority over settled populations residing within defined geographic boundaries. The Treaty of Westphalia (1648) was a critical turning point in establishing that new landscape, but for the next two centuries individuals still enjoyed substantial freedom in traversing the boundaries of the State. This reflected an attitude of hospitality to strangers that was inherited from ancient cultures and expressed through cosmopolitanism. This attitude can be seen in the scholarly works of the great writers of international law of the eighteenth and nineteenth centuries, who, with few exceptions, took a liberal attitude to the movement of people across borders for trade, commerce and other purposes. If law in this early period paid scant attention to regulating migration, it was partly for the practical reason that the number of people involved was modest because modes of transport were confined to land crossings by foot or horse, and sea crossings by wind-powered sailing vessels. There were some notable exceptions to this laissez-faire approach, but the circumstances in which law was invoked to regulate international migration were few.