Are video game emulators illegal

[Ahem]. This matter has been talked about many time and there are probably many questions like this on this site. The thing is that emulators are programs made with a copied code from the actual system or from scratch with similar codes. As far as I know, they are legal as long as you don't sell them, buy them, or screw up the actual sysytems (like a N64(c) emulator on Xbox360(c)).