On Sept. 15, 2008, Lehman Brothers, one of the biggest investment banks in the world, filed for bankruptcy. How did it happen, and how did it affect the economy? Here’s what you need to know: 1. What’s Lehman Brothers, anyway? Before filing for bankruptcy in September 2008, Lehman Brothers was the fourth-largest investment bank in the U.S. (Goldman Sachs, Morgan Stanley and Merrill Lynch were first, second and third, respectively). It was founded in 1850 and did business for 158 years.