The Mumbai Metro is a rapid transit system under construction in the Indian city of Mumbai. The system is designed to reduce traffic congestion in the city, and will be built in three phases over a 15-year period, with overall completion expected in 2021. The Mumbai Metro's operator is Mumbai Metro One Pvt Ltd (MMOPL), a joint venture company formed by Reliance Infrastructure, Veolia Transport and the Mumbai Metropolitan Region Development Authority (MMRDA).

In June 2006, Prime Minister of India Manmohan Singh inaugurated the first phase of the Mumbai Metro project, which will ultimately comprise a three-line high-capacity metro railway system spanning 63 kilometres (39 mi). Construction work began in February 2008, and the system's first line is expected to be completed in March 2013, although some aspects of the project have been afflicted by delays and cost issues.

Greater Mumbai is the financial capital of India and the heart of its commercial and trade activities. It is also among the largest cities in the world, with a total metropolitan area population of over 20 million as of 2011, and a population growth rate of around 2% per annum. Mumbai has the advantage of a high modal share of the public (88%) in favour of a public mass transport system. The existing Mumbai Suburban Railway carries over 7 million passengers per day, and is supplemented by the BEST bus system, which provides feeder services to station-going passengers to allow them to complete their journeys. However, due to the city’s geographical constraints and rapid population growth, road and rail infrastructure development has not been able to keep pace with growing demand over the past several decades. Moreover, the Mumbai Suburban Railway, though extensive, is not built to rapid transit specifications.

Mumbai /mʊmˈbaɪ/, also known as Bombay /bɒmˈbeɪ/, is the capital of the Indian state of Maharashtra. It is the most populous city in India, and the fourth most populous city in the world, with a total metropolitan area population of approximately 20.5 million. Along with the neighbouring urban areas, including the cities of Navi Mumbai and Thane, it is one of the most populous urban regions in the world. Mumbai lies on the west coast of India and has a deep natural harbour. In 2009, Mumbai was named an Alpha world city. Mumbai is also the richest city in India, and has the highest GDP of any city in South, West or Central Asia.

The seven islands that came to constitute Mumbai were home to communities of fishing colonies. For centuries, the islands were under the control of successive indigenous empires before being ceded to the Portuguese and subsequently to the British East India Company. During the mid-18th century, Mumbai was reshaped by the Hornby Vellard project, which undertook the reclamation of the area between the seven constituent islands from the sea. Completed by 1845, the project along with construction of major roads and railways transformed Bombay into a major seaport on the Arabian Sea. Economic and educational development characterised the city during the 19th century. It became a strong base for the Indian independence movement during the early 20th century. When India became independent in 1947, the city was incorporated into Bombay State. In 1960, following the Samyukta Maharashtra movement, a new state of Maharashtra was created with Bombay as capital. The city was renamed Mumbai in 1996.