A better understanding of string theory can be gained by
exploring its behaviour at high energies. It is in this limit that,
as the inelastic scattering channels grow in importance and variety,
the extended nature of the string becomes manifest. In the same limit,
the strength of the gravitational interactions is also enhanced and
we can address some of the fundamental problems posed by a theory
of quantum gravity.

I will study a simple system, a highly-energetic closed string colliding
with a stack of parallel D-branes. Analyzing this process, we will be
able to explore several  aspects of string and gravitational
interactions: the emergence of classical gravitational effects from the string
S-matrix in flat spacetime; the dynamical excitation of the massive, 
higher-spin states of the string spectrum; the existence of a unitary 
description for the infall of a string in a spacetime singularity. 
I will also discuss how causality and the behaviour of the scattering 
amplitudes in the Regge limit constraint the higher derivative corrections 
to general relativity.