  These notes provide a hopefully pedagogic introduction to Born-Infeld theory
and some of its uses in String/M-Theory from the perspective of some-one
interested in spacetime geometry. Causality and the consequences of having both
the open string and closed string metric are stressed. This leads to some new
insighst into old problems, such as strings at finite temperature. Various
strong coupling limits of both Born-Infeld and the M5-brane theories are
described and their relevance for tachyon condensations are mentioned.
