During the nineteenth and twentieth centuries, the intensity, fluctuation, and distance of 'proletarian mass migrations' grew steadily. Apart from permanent immigration into industrial areas and employments, temporary and seasonal mass migrations took place, partly as transitional phenomena. They only partly moved within the traditional migratory systems which on the eve of the age of industrialization became replaced by these new movements, including millions of migrants. Their employment in agrarian production as well as in the service sector added to the rapidly expanding urban areas and industrial agglomerations which, to a large extent, were built by migrant workers, too. Moreover, the large and moving railway, road, bridge, and tunnel construction sites attracted a highly mobile migrant workforce. In addition to the ‘proletarian mass migrations,’ individually migrating experts e.g. from Great Britain functioned as a sort of industrial development workers, like the 'puddlers' in early steel production on the continent. And there were travelling entrepreneurs heading especially to Great Britain, an in-between of education travel and industrial espionage scouting for new machines as well as industrial processes.