India is home to 25 of the 100 fastest growing cities in the world. This growth, to a great extent, is fuelled by rural-to-urban migration. While the rural-urban divide in employment has been one of the primary reasons for labour mobility, everything is not hunky-dory in the cities. The pressure on accommodation owing to migration is the main reason for India's swelling slums. In fact, a quarter of the population of India's 19 biggest cities live in slums. Mumbai, Delhi and Kolkata are the largest destinations of internal migrants. The largest migrant-sending states are Uttar Pradesh, Bihar and Rajasthan.