The historical study of migration from Ireland is striking in that little or no heed is paid to the vast and burgeoning literature on possible theoretical frameworks which attempt to explain and understand this phenomenon in many analyses – both historical and contemporary – of Irish migration. Even though Irish migration would appear on first inspection to conform to general models of migration which view the movement of people as part of the natural process whereby labour is directed to those areas or regions that require large numbers of people to sustain economic growth, this is far from the complete story. Equally Irish migration may well also be interpreted as a flow of people from the periphery to the economic core zones of the United States, Britain and other diverse places, which reinforced the uneven nature of capitalist economic development in the nineteenth and twentieth centuries. Needless to say no one all-encompassing model neatly explains migration from modern Ireland. However, the benefits in reviewing theoretical approaches to the study of migration are twofold. Firstly, migration theory raises many intriguing lines of inquiry, and the principal objective of this chapter is to illustrate briefly the scope and range of migration theory and in doing so to assess the possibilities of its application to the evidence available for twentieth-century Ireland. Secondly, the existence of this body of research underlines the obvious point that migration is a global process that has shaped the evolution of most societies in the modern world, albeit to varying degrees.