  The fivebrane of M theory -- the M5-brane -- is an especially interesting
object. It plays a central role in a geometric understanding of the
Seiberg-Witten solution of N=2 D=4 gauge theories as well as in certain new 6d
quantum theories. The low energy effective action is an interacting theory of a
(2,0) tensor multiplet. The fact that this multiplet contains a two-form gauge
field with a self-dual field strength poses special challenges. Recent progress
in addressing those challenges is reviewed.
