A common question asked on the Video and Online Games section is “Where can I get an emulator and ROMs for <insert game system here>?” The legality of emulators and ROMs are often confused, so I thought I’d lay it out here. I am not a lawyer, so don’t take this as legal advice. This is just the law as I understand it.
Emulators are programs that make a machine function like another machine. For example, ZSNES is a program that will make your computer act like a Super Nintendo (SNES). These are perfectly legal in every case. In the case SONY v. CONNECTIX it was the decision of the court that CONNECTIX had the right to emulate the SONY Playstation, and that reverse engineering of the Playstation BIOS for such a cause fell under fair usage. As a result, emulation of consoles and handhelds is completely legal. This explains why emulators are widely available from a multitude of sources (for example, SourceForge has a few, I know for sure they have them for the NES, SNES, and N64)
ROM stands for Read Only Memory. In this case, a ROM contains all the information normally placed on a cartidge or disc depending on what media the console you are trying to emulate. The legality of these are tricky.
Section 117 of the US Copyright Law allows legal copies of software to be made, provided that you follow specific guidelines, namely: (1) You are making the copy for backup (a.k.a. “archival” in the law) purposes only, (2) you are the legal owner of the original or have been authorized to make the copy for them (read: not for yourself), and (3) if you decide to sell, trade, or give away the original, the copy must be immediately destroyed or transferred to the new owner. In addition to these guidelines, any tools that are used to break a copy-protection are illegal by the Digital Millenium Copyright Act of 1998 (pdf), and thus any copies made using these tools are therefore also illegal. It is these laws that make most ROMs illegal to possess, and certainly illegal to download (hence the difficulty many people have in finding them).
It is worth noting that there are some ROMs that are freeware or in the public domain, and these are perfectly legal to download and share with others. In addition, some developers create “homebrew” applications for use with consoles (like the Wii, PSP, and DS) which often come in the form of a ROM. These are also completely legal to download, use, and share.
So, if you are looking to get into emulation of older consoles and enjoy nostalgic games, I encourage you to do so. Just keep in mind the legality (or lack thereof) of the ROMs you are obtaining.