  We explain how structures related to octonions are ubiquitous in M-theory.
All the exceptional Lie groups, and the projective Cayley line and plane appear
in M-theory. Exceptional G_2-holonomy manifolds show up as compactifying
spaces, and are related to the M2 Brane and 3-form. We review this evidence,
which comes from the initial 11-dim structures. Relations between these objects
are stressed, when extant and understood. We argue for the necessity of a
better understanding of the role of the octonions themselves (in particular
non-associativity) in M-theory.
