In-ear monitors or IEM's are earphones used by musicians during a concert or live show. They are also used by DJs sometimes. The best thing about IEMs is the level of noise isolation they provide. They fit directly into the ear canal and block the surrounding noises from entering even in the noisest of the environments. IEM's let musicians hear themselves along with the instruments that are being played. This way they can monitor themselves without keeping any large speakers for themselves. Also, IEMs serve a completely different purpose, we don't think they could be used as mixing headphones.