There are currently (5) artists using the name "Theory" (1) Hip-Hop artist from Boston, MA. Part of Merk Productions. (2010-current)
(2) UK based Drum'n'bass (Jungle) producer (2005 - current)
(3) Hip-Hop artist from Yonkers,New York, USA (1998 - current)
(4) Hip-Hop/Folk/other solo artist from Brighton, UK (1994 - Current) Now "Theory 77".
(5) Hybrid Rock/Metal band from New Jersey, USA (2000 - 2003)

Read more about Theory on Last.fm.