- There are two basic versions of the Sega CD - the CD-X standalone unit and an add-on unit tailored to plug into the rightside expansion slot of a Genesis/MegaDrive console. The original version of the add-on unit goes under the console, whereas the retooled version sits beside it. The Genesis 3 console by Majesco has no expansion slot and is therefore incompatable with the add-on unit.

- The JVC X-Eye, aka the Wonder Mega, was an oversized clone of the CD-X unit authorized by Sega. This is because JVC made the CD players that Sega used in its own units. They work very well, and are the closest thing you will find to an all-in-one Sega 16-bit console outside of the hard-to-find CD-X (sorry, no built-in 32X mode as was planned for the abandoned Sega Neptune). It has come to my attention that the Japanese version was designed to be used with some of the early karaoke discs; I have no idea if the export version would do this as well. The X-Eye has become a rare find nowadays, so if you're a G/MD game collector and see one loose, grab it!

- There is a European variation of the Sega CD known as the MultiMega. It generally resembled a Mark 2 MegaDrive console but included a built-in Mega CD player. Not much is known about it at this time.

- There is yet another unit that can handle Sega CD games, and this one comes from Pioneer. The Laservision CLD-A160 is a combination laserdisc and videogame unit that has a plug-in bay at the bottom for various home console units, while the disc player is employed for those consoles that support CD games. Both MegaDrive/Mega CD and PC Engine modules were made for it, and I'm told there were others as well. Again, these are quite rare and make exellent 16-bit combo units, but they come at a rather steep price (US$300 the last time I checked, and that was in my local pawn shop).