Traditional homes aren't framed with metal studs, but office buildings and some newer condos are. Hanging a mirror on metal studs takes a bit of preparation and know-how. Typical wood screws (or even self-tapping screws) won't cut it, because metal studs are only about 1/16 inch thick, and in the long run your mirror could fall. Mirrors come with standard hardware, and the mirror itself will have either mounting holes on the side or brackets on the back.
Things You'll Need
Electric drill and bits
Take the mirror hardware (that came standard with your mirror) and visit your local hardware store. You will need to buy toggle bolts to mount to the metal studs. Toggle bolts go behind the stud and spread out to handle the pressure. Make sure the heads of the toggle bolts you get are the same size or slightly larger than the mirror's original screws.
Use a stud finder to find the metal studs behind the wall. Office buildings and condos vary on spacing but can range from 16 to 24 inches on center. Mark each stud's position with a pencil. Make sure to have at least two studs supporting your mirror. Toggle bolts can go into drywall, too, if the spacing requires it.
Drill pilot holes using an electric drill and bits. You will need to drill holes large enough to fit a toggle bolt directly through each hole and the stud behind it. Use a level (or a measuring tape if you don't have one) to make sure the holes will fit the mirror square.
Insert the bolts into each hole and tighten them up. If the mirror hardware has brackets that need to be mounted behind the screws, put them on before putting the bolts into the holes. Some mirrors have instructions that direct you put the bolts in, than hang the mirror on brackets that are already on the mirror.
Mount the mirror on the toggle bolts or brackets and tighten them by hand. Don't over-tighten the bolts, as this will cause unnecessary pressure.