Urbaniz.ation is an important social phenomenon that involves an array of interrelated process of demographic, economic, environmental and technological changes. The twentieth century witnessed a rapid urbanisation of the world's population. The global proportion of urban population increased from a mere 13 per cent in 1900 to 29 per cent in 1950 and, reached 49 per cent in 2005. The rising numbers of urban dwellers give the best indication of the scale of these unprecedented trends: the urban population increased from 220 million in 1900 to 732 million in 1950, and reached 3.2 billion in 2005, thus more than quadrupling since 1950. In the middle of the year 2009, the number of people living in urban areas (3.42 billion) surpassed the number living in rural areas (3.41 billion), thus more than quadrupling since 1950 . Since then the world has become more urban than rural. 'Urban' refers to an area where a multiplicity of communities of substantial size (minimum population of 5,000) and density (at least 400 people per sq km) live with a variety of non-agricultural pursuits, with good infrastructure facilities.However, this urban expansion is not a phenomenon of wealthy countries. Major part of the growth has occurred in unplanned and under-served city slums. The pace of urbanization exceeds the rate at which basic infrastructure and services can be provided, and the consequences for the urban poor have been dire. At present more than 32 percent of the world's urban population are estimated to live in slums. Failure to prepare for this unprecedented and inevitable urban explosion carries serious implications for global security and environmental sustainability.