  The recently found non-critical open string theories is reviewed. These open
strings, noncommutative open string theories (NCOS), arise as consistent
quantum theories describing the low energy theory of D-branes in a background
electric B-field in the critical limit. Focusing on the D3-brane case, we
construct the most general (3+1) NCOS, which is described by four parameters.
We study S and T -dualities of these theories and argue the existence of a
U-duality group.
