Industrialisation and economic development have always led to labour migration. The initial form is rural-urban migration, but where labour demand outstrips the availability of local workers, this is often followed by international movements. The European industrial revolutions of the 18th and 19th centuries were marked by such migrations: from Ireland to Britain, from Poland to Germany, from Italy to France and Switzerland. Between 1850 and 1914 the economic development of the USA led to enormous migrations from Europe. From the 1920s to the 1960s, workers for the North, Middle-West and West were provided by the ‘Great Migration’ of black people from the former slave states of the South. Migration has always been a major concern for the trade union movement, presenting both problems and new impulses. In the USA the employers’ use of successive waves of new immigrants as cheap labour and strike-breakers proved a serious threat.