  In this review we describe our current understanding of the properties of
open string tachyons on an unstable D-brane or brane-antibrane system in string
theory. The various string theoretic methods used for this study include
techniques of two dimensional conformal field theory, open string field theory,
boundary string field theory, non-commutative solitons etc. We also describe
various attempts to understand these results using field theoretic methods.
These field theory models include toy models like singular potential models and
p-adic string theory, as well as more realistic version of the tachyon
effective action based on Dirac-Born-Infeld type action. Finally we study
closed string background produced by the `decaying' unstable D-branes, both in
the critical string theory and in the two dimensional string theory, and
describe the open string completeness conjecture that emerges out of this
study. According to this conjecture the quantum dynamics of an unstable D-brane
system is described by an internally consistent quantum open string field
theory without any need to couple the system to closed strings. Each such
system can be regarded as a part of the `hologram' describing the full string
theory.
