  Following Sen's discovery of various stable non-BPS D-branes, K-theory has
been shown to be the appropriate mathematical framework for classifying
conserved D-brane charges. The classification accounts for known D-branes and
predicts some new ones including a D8-brane in type I superstring theory. After
briefly reviewing these developments, we discuss certain issues pertaining to
the D8-brane, which is unstable.
