Remarks on non-BPS D-branes

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