Delhi has earned the reputation of the fourth most polluted city in the world. The urbanization and industrialization had their share of air pollution but vehicles on Delhi roads drove the last nail in the coffin. The vehicle registration data reveals that Delhi has more vehicles than the other three metros Bombay, Kolkata and Chennai taken together. No doubt, on the economic front, it was prosperity but on the environmental front it was a disaster Trucks, buses, MUVs, cars, 3 wheelers and 2 wheelers of all vintages running on the roads were belching out smoke, making Delhi air unbreathable. The poor fuel quality and traffic management made things much worse. You could take any parameter for air quality check: CO2, NOx, SO2, PM, etc., it was beyond the prescribed limit. Airborne diseases did not spare anyone - children and older people were the worst sufferers. Press and media made a big hue and cry but all the voices fell on deaf ears till the Supreme Court intervened in the matter. Suddenly all concerned became active, the legislature, the government, the automobile manufacturers, the fuel producers, fleet owners, commuters and the last but not the least the citizens of Delhi at large who had to face both the economic and environmental brunt. The article takes a stock of air quality of Delhi and describes the process of transformation which led Delhi to be recognized for its effort to clean the ambient air, resulting in a cleaner environment.