Where's the MAF Sensor located?

I see several post concerning replacing the Mass Air Flow sensor to correct whining tranismissions. Where is it located? How difficult is it to replace?

Art,

MAF is attached to the air filter housing. I would describe it as a "square looking thing" in the air duct between the air filter housing and the intake. I have never had to replace it, so I don't know how tough it would be to replace.

Thanks.....