Johannesburg (pronounced /jō-hān'ĭs-bûrg'/) is the largest and most populous city in South Africa. Johannesburg is the provincial capital of Gauteng, the wealthiest province in South Africa, having the largest economy of any metropolitan region in Sub-Saharan Africa. The city is one of the 40 largest metropolitan areas in the world (), it is Africa's most advanced city, and one of Africa's only two global cities, the other being Cairo according to the Globalization and World Cities group's 1999 inventory, which classified it as a gamma world city). While Johannesburg does not form one of South Africa's three capital cities, it does house the Constitutional Court – South Africa's highest court.
Johannesburg is the source of a large-scale gold and diamond trade, due to its location on the mineral-rich Witwatersrand range of hills. Johannesburg is served by O.R. Tambo International Airport, the largest and busiest airport in Africa and a gateway for international air travel to and from the rest of southern Africa.
According to the 2007 Community Survey, The population of the Greater Johannesburg Metropolitan Area is almost eight million. The population of the municipal city is almost 4 million. Johannesburg's land area of 1,645 square kilometres (635 sq mi) is very large when compared to other cities, resulting in a population density of 2,364 inhabitants per square kilometre (6,123/sq mi). Johannesburg also encompasses Soweto to the south west, a township that the apartheid government established to accommodate the large number of migrant workers. It should be noted that Johannesburg and Pretoria are beginning to act as one functional entity, connecting the province of Gauteng together and forming one Megacity of roughly 10 million people.
Gauteng (as a city) is growing rapidly, due to mass urbanisation that is a feature of many developing countries. According to the State of the Cities Report, the urban portion of Gauteng – comprised primarily of the cities of Johannesburg, Ekurhuleni (the East Rand) and Tshwane (greater Pretoria) – will be a polycentric urban region with a projected population of some 14.6 million people by 2015, making it one of the largest cities in the world.