  We study the dynamics of ultrarelativistic D-branes. The dominant phenomenon
is bremsstrahlung: mild acceleration induced by closed string interactions
triggers extremely rapid energy loss through radiation of massless closed
strings. After characterizing bremsstrahlung from a general k-dimensional
extended object in a D-dimensional spacetime, we incorporate effects specific
to D-branes, including velocity-dependent forces and open string pair creation.
We then show that dissipation due to bremsstrahlung can substantially alter the
dynamics in DBI inflation.
