Johannesburg is one of the world's leading financial centres and it is the economic and financial hub of South Africa, producing 16% of South Africa's gross domestic product, and accounts for 40% of Gauteng's economic activity. In a 2008 survey conducted by MasterCard, Johannesburg ranked 47 out of 50 top cities in the world as a worldwide centre of commerce (the only city in Africa)
Mining was the foundation of the Witwatersrand's economy, but its importance is gradually declining due to dwindling reserves and service and manufacturing industries have become more significant to the city's economy. While gold mining no longer takes place within the city limits, most mining companies still have their headquarters in Johannesburg. The city's manufacturing industries extend across a range of areas and there is still a reliance on heavy industries including steel and cement plants.
The service and other industries include banking, IT, real estate, transport, broadcast and print media, private health care, transport and a vibrant leisure and consumer retail market. Johannesburg has Africa's largest stock exchange, the JSE although it has moved out of the central business district. Due to its commercial role, the city is the seat of the provincial government and the site of a number of government branch offices, as well as consular offices and other institutions.
The Witwatersrand urban complex is a major consumer of water in a dry region. Its continued economic and population growth has depended on schemes to divert water from other regions of South Africa and from the highlands of Lesotho, the biggest of which is the Lesotho Highlands Water Project, but additional sources will be needed early in the 21st century.
The container terminal at City Deep is known to be the largest "dry port" in the world, with some 50% of cargo that arrives through the ports of Durban and Cape Town arriving in Johannesburg. The City Deep area has been declared an IDZ (industrial development zone) by the Gauteng government.