  We discuss how the BPS branes of M-theory could be described as bound states
of non-BPS M10-branes. This conjectured M10-brane is constructed as an unstable
spacetime-filling brane in the massive eleven dimensional supergravity defined
with a Killing direction, such that 1) the BPS M9-brane is obtained after the
tachyonic mode of an M2-brane ending on it condenses, and 2) it gives the
non-BPS D9-brane of the type IIA theory upon reduction. The existence of other
non-BPS M-branes is also discussed, together with their possible stabilisation
within the Horava-Witten construction.
