text: what is a money market mutual fund? what are risks? what are avg. returns? what's in it for them? what terms?   money market mutuals funds are invested in a variety of short term corporate & government notes. they fund takes a small percentage for managing the money. some money market funds are us treasury only funds and invest in government obligations only. some invest only in municipal bonds to gain the tax free status.  the risk is that some of the investments default and the share price is not held at a constant $1.00.. you would lose money in that case. most major fund companies would never let that happen to a money market fund as it would kill off most of their business. they have and would add the extra cash to keep the net asset value at $1.00.  moral of the story, buy money market funds from the best companies, vanguard.com, fidelity.com, troweprice.com, schwab.com ,etc.