  The ultrarelativistic limit of the Kerr - Newman geometry is studied in
detail. We find the gravitational shock wave background associated with this
limit. We study the scattering of scalar fields in the gravitational shock wave
geometries and compare this with the scattering by ultrarelativistic extended
sources and with the scattering of fundamental strings.
  We also study planckian energy string collisions in flat spacetime as the
scattering of a string in the effective curved background produced by the
others as the impact parameter $b$ decreases. We find the effective energy
density distribution generated by these collisions. The effective metric
generated by these collisions is a gravitational shock wave with profile
$f(\rho)\sim p\rho^{4-D}$, for large impact parameter $b$. For intermediate
$b$, $f(\rho)\sim q\rho^2$, corresponding to an extended source of momentum
$q$.
  We finally study the emergence of string instabilities in $D$ - dimensional
black hole spacetimes and De Sitter space. We solve the first order string
fluctuations around the center of mass motion at spatial infinity, near the
horizon and at the spacetime singularity. We find that the time components are
always well behaved in the three regions and in the three backgrounds. The
radial components are unstable: imaginary frequencies develop in the
oscillatory modes near the horizon, and the evolution is like $(\tau-\tau_0)^
{-P}$, $(P>0)$, near the spacetime singularity, $r\to0$, where the world -
sheet time $(\tau-\tau_0)\to0$, and the proper string length grows infinitely.
