The following is taken from http://www.mameworld.net/easyemu/mamechd.htm A CHD file is a compressed image of the hard drive used in an arcade cabinet. Hard drives are used in arcade games because of the huge storage available on them (gigabytes!). Luckily for us the lovely Mame developers have found a way to support a compressed version of these hard drives although some can still be quite large. At the moment there are now several games that use hard drives with sizes from 50MB up to over 2GB so make sure you have plenty of space left. -------------------------------------------------------------------------------- Where to put them! CHD files need to be put in their own folders in you main Roms folder. For example Killer Instinct uses a romfile called kinst.zip and a hard disk file called kinst.chd. First the rom file need to be put (Still zipped!) into the main Mame roms folder (Normally C:MameRoms). You then need to create a new folder in your roms folder called kinst and copy the kinst.chd (Not zipped!) file into it (C:MameRomskinst). If you are unsure how to create folders then have a look at the Windows Basics page as it has a description of what to do.