The world is becoming an increasingly urban place. Current estimates indicate that one half of the world's population will be living in urban centers by the year 2005, largely the result of but two centuries of rapid urbanisation, and there are no apparent social forces that strongly oppose it.

Urbanism involves the concentrated inhabitation of human populations in relatively small areas, while urbanisation is the process of becoming urban and includes population growth by migration, natural increase and the changing scale of economic activity associated with this change. Both influence human biology and behavior in ways distinct from rural lifeways. Humans, like all other living things, are affected by their environment, whether that environment is completely unaltered by human activity or made entirely from it. Evolution and adaptation continue to operate in human populations through differentials in morbidity, mortality and reproduction.