  We review the recent developments in our understanding of non-BPS states and
branes in string theory. The topics include 1) construction of unstable non-BPS
D-branes in type IIA and type IIB string theories, 2) construction of stable
non-BPS D-branes on various orbifolds and orientifolds of type II string
theories, 3) description of BPS and non-BPS D-branes as tachyonic soliton
solutions on brane-antibrane pair of higher dimension, and 4) study of the
spectrum of non-BPS states and branes on a system of coincident D-brane -
orientifold plane system. Some other related results are also discussed
briefly.
