We find the analog of the Boillat metric of Born-Infeld theory for the M5-b
rane. We show that it provides the propagation cone of all 5-brane degrees.
In an arbitrary background field, this cone never lies outside the Einstei
n cone. An energy momentum tensor for the three-form is defined and shown t
o satisfy the Dominant Energy Condition. The theory is shown to be well def
ined 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 s
how that the corresponding theory is conformally invariant and admits infin
itely many conservation laws. On reduction to the Born-Infeld case this agr
ees with the work of Bialynicki-Birula. (C) 2001 American Institute of Phys
ics.