Urbanization is the process in which large numbers of people concentrate their residences in relatively small areas, thus forming cities. It is a result of the strong desire by many people to live in close proximity to large numbers of other people, and it has been made possible mainly by increases in agricultural productivity.
Urbanization has been occurring for thousands of years but has accelerated in the past several centuries as a consequence of rapid advances in agricultural technology so that the percentage of the world's population living in urban areas has soared from well under five percent at the start of the nineteenth century to more than fifty percent by the early 21st century.
Reasons for this strong desire by many people to live close to large numbers of other people despite the often substantial disadvantages (e.g., air pollution and noise pollution) can include more and better employment and business opportunities, a greater choice of potential mates, better educational opportunities, more intellectual stimulation, improved healthcare, and better safety.