  We discuss instanton operators in five-dimensional gauge theories. These are
defined as disorder operators which create a non-vanishing second Chern class
on a four-sphere surrounding their insertion point. As such they may be thought
of as higher-dimensional analogues of three-dimensional monopole (or `t Hooft)
operators. We argue that they play an important role in the enhancement of the
Lorentz symmetry for maximally supersymmetric Yang-Mills to SO(1,5) at strong
coupling.
