  We review the recent studies of tachyon condensation in string field theory.
After introducing the open string field theory both for bosonic string and for
superstring, we use them to examine the conjecture that the unstable
configurations of the D-brane will decay into the `closed string vacuum'
through the tachyon condensation. And we describe the attemps to construct a
lower dimensional bosonic D-brane as an unstable lump solution of the string
field equation. We obtain exact results from another formulation, background
independent open string field theory. We also discuss some other topics which
are related to tachyon condensation in string theory, such as the construction
of a D-brane as a noncommutative soliton and some field theory models. This
paper is based on my master's thesis submitted to Department of Physics,
Faculty of Science, University of Tokyo on January 2001.

High Energy Physics - Theory