  String duality requires the presence of solitonic $p$-branes. By contrast,
the existence of fundamental supermembranes is problematic, since they are
probably unstable. In this paper, we re-examine the quantum stability of
fundamental supermembranes in 11 dimensions. Previously, supermembranes were
shown to be unstable by approximating them with SU(n) super Yang-Mills fields
as $n \rightarrow \infty$. We show that this instability persists even if we
quantize the continuum theory from the very beginning. Second, we speculate how
a theory of decaying membranes may still be physically relevant. We present
some heuristic arguments showing that light membranes may be too short-lived to
be detected, while very massive membranes, with longer lifetimes, may have only
very weak coupling to the particles we see in nature. Either way, decaying
membranes would not be detectable in the lab. (This article is to be published
in Frontiers in Quantum Field Theory (World Scientific) dedicated to Keiji
Kikkawa's 60th birthday.)
