A city is a large and permanent settlement containing complex systems for sanitation, utilities, land usage, housing, and transportation. We saw a fast rise in the number and sizes of cities beginning with the 20th century. By the end of the century, more than 50% of humankind is living in metropolitan areas. In industrialized countries it is 70%. Of course this fast rise in our urban population also brought many challenges: Cities in developing countries and newly-industrialized countries are struggling to catch up with the pace. The result are huge slums that have no access to electricity, sanitation, and even clean water! Examples are Rio de Janeiro, Lagos and Cairo. But we cannot deny the positive side of urbanization.
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