The history of the Bronx during the 20th century may be divided into four periods: a boom period during 1900–29, with a population growth by a factor of six from 200,000 in 1900 to 1.3 million in 1930. The Great Depression and post World War II years saw a slowing of growth leading into an eventual decline. The mid to late century were hard times, as the Bronx declined 1950–85 from a predominantly moderate-income to a predominantly lower-income area with high rates of violent crime and poverty. The Bronx has experienced an economic and developmental resurgence starting in the late 1980s that continues into today.

When was the Bronx's boom period?