On March 17 of 1930, on the site of the old Waldorf-Astoria Hotel, the construction of the Empire State Building was begun. The preliminary excavation had begun on January 22, even before the old Waldorf-Astoria was completely demolished. The site took up 83,860 square feet of land, with the building firm of Starrett Brothers & Eken, Inc labouring for 14 months to complete the massive skyscraper, employing a workforce of up to 3400 people.
Total building time was 7 million man hours, 1 year and 45 days work, including Sundays and holidays. Progress was rapid, with the framework rising at the rate of 4 1/2 stories per week.
When it was completed in May, 1931, the Empire State Building, at 102 stories, was the tallest building in the world until the completion of the first tower of the World Trade Center in Lower Manhattan in 1972.
The project was expected to cost in excess of $50 million, but due to the Great Depression building was completed for "only" $24,718,000 (1931 dollars). The additional cost of the land brought total costs up to $40,948,900 (1931 dollars).
The cornerstone was laid on September 17, 1930 by Alfred E. Smith, former governor of New York and the official opening ceremony was held on May 1, 1931 with President Herbert Hoover pressing a button in Washington, D.C. to turn on the building's lights.
note that this writeup is meant to fit with perdedors excellent writeup. It is designed to minimize redundancy and therefore does not restate things he has already covered<
Some sources for this writeup include the official Empire State Building homepage at http://www.esbnyc.com/ as well as http://www.greatbuildings.com/buildings/Empire_State_Building.html