One of the outstanding phenomena of the 20th century was the huge migrations of population it witnessed. It is natural to think first of the effects of war, exacerbated by nationalism and beliefs about ethnicity. A major example is the forced displacement of millions of people in Europe and the Middle East in 1945 and afterwards. But these movements are dwarfed by the migration of rural dwellers to urban areas, a process with origins in the industrial revolution but which in the past 60 years has turned from a stream to a tsunami. (Commentary)