After Sega established a strong lead in the console market in the early 1990's, the company saw the possibilities that the new CD-ROM format could bring to gaming. Sega produced the Sega CD add-on for the Genesis to bring the technology to the many Genesis owners around the world without requiring them to invest in a completely new system. Unfortunately, they did not completely follow through with their vision.
Even though it was not a commercial success, that does not mean that a Sega CD isn't worth picking up for a reasonable price to expand on one's classic Sega collection. The Sega CD is still an interesting piece of hardware and has some games that are still worth checking out.