  We find the analogue of the Boillat metric of Born-Infeld theory for the
M5-brane. We show that it provides the propagation cone of {\sl all} 5-brane
degrees. In an arbitrary background field, this cone never lies outside the
Einstein cone. An energy momentum tensor for the three-form is defined and
shown to satisfy the Dominant Energy Condition. The theory is shown to be well
defined for all values of the magnetic field but there is a limiting electric
field strength. We consider the strong coupling limit of the M5-brane and show
that the corresponding theory is conformally invariant and admits infinitely
many conservation laws. On reduction to the Born-Infeld case this agrees with
the work of Bia{\l}nicki-Birula.
