Well when you buy a house and get a mortgage, that mortgage company takes your mortgage at 6% and bundels it with 1000's of other mortgages with similar credit and interest rates and sells them to whomever (pension funds, bond mutual funds, banks, overseas investors, etc) as a bond paying 6%. These bonds traded freely throughout the market for years. Once the housing market started to turn south, and people stopped paying their mortgage, those pension/mutual funds/banks had to do something with the bonds. Due to accounting rules, they are required to check the market (aka "Mark to Market") what the bonds are worth. For the past several months that market doesnt exist anymore since the only ones who really had mortgage backed securites trading floors were Bear Sterns and Lehman brother (Other do but not as big as BS+LEH) Since no market exists, those bonds are essentially worthless. Now here is the problem, those bonds were backed by Freddie and Fannie and were deemed good quailty (most still are good quaility) and were used as collateral to buy more MBS or to finance other deals, some were even used for money markets.