We review recent developments in the construction of heterotic and type II
string field theories and their various applications. These include systematic
procedures for determining the shifts in the vacuum expectation values of
fields under quantum corrections, computing renormalized masses and S-matrix of
the theory around the shifted vacuum and a proof of unitarity of the S-matrix.
The S-matrix computed this way is free from all divergences when there are more
than 4 non-compact space-time dimensions, but suffers from the usual infrared
divergences when the number of non-compact space-time dimensions is 4 or less.