Harlem is a neighborhood in the New York City borough of Manhattan, which since the 1920s has been a major African-American residential, cultural, and business center. Originally a Dutch village, formally organized in 1658,  it is named after the city of Haarlem in the Netherlands. Harlem was annexed to New York City in 1873.
Harlem has been defined by a series of boom-and-bust cycles, with significant ethnic shifts accompanying each cycle. Black residents began to arrive en masse in 1904, with numbers fed by the Great Migration. In the 1920s and 1930s, the neighborhood was the locus of the “Harlem Renaissance”, an outpouring of artistic and professional works without precedent in the American black community. However, first, with the job losses in the time of the Great Depression and especially after World War II with deindustrialization in New York, rates of crime and poverty increased significantly.After four decades of decline, Harlem’s population bottomed out in the 1990 census, at 101,026. It had decreased by 57% from its peak of 237,468 in 1950. Between 1990 and 2006 the neighborhood’s population grew by 16.9%, including new middle-class residents of African-American, European-American, Hispanic and Asian descent. 
New York’s revival in the late 20th century has led to renewal in Harlem as well. By 1995, Harlem was experiencing social and economic gentrification. This was driven by changing federal and city policies, including fierce crime-fighting and a concerted effort to develop the retail corridor on 125th Street. Around the same time, the Upper Manhattan Empowerment Zone funneled money into new developments.
The number of housing units in Harlem increased 14% between 1990 and 2000 and the rate of increase has been much more rapid in recent years. Property values in Central Harlem increased nearly 300% during the 1990s, while the rest of the City saw only a 12% increase. 
Even empty shells of buildings in the neighborhood were, in of 2007, routinely selling for nearly $1,000,000 each.  These days, shells range anywhere from $1,400,000 – $1,900,000.
 Pierce, Carl Horton, et al. New Harlem Past and Present: the Story of an Amazing Civic Wrong, Now at Last to be Righted. New York: New Harlem Pub. Co., 1903.
 “Harlem’s Shifting Population” by Andrew Beveridge August 27, 2008
 The Economic Redevelopment of Harlem, PhD Thesis of Eldad Gothelf, submitted to Columbia University in May 2004
 “After the Shell Game”, S. Jhoanna Robledo, New York Magazine, March 26, 2007, p.69. The article states that, after rocketing upwards for many years, prices on shells have settled to about the same level in 2007 as they had been in 2005. Examples are given of sales around $800,000.