  This thesis examines the interaction of both bosonic and superstrings with
various backgrounds with a view to understanding the interplay between tachyon
condensation and world-sheet conformal invariance, and to understanding the
overlap of d-branes and closed string modes. We develop the boundary state and
show that in a background of interest to tachyon condensation the conformal
invariance of the string world-sheet is broken, which suggests a generalized
boundary state obtained by integrating over the conformal group of the disk. We
find that this prescription reproduces particle emission amplitudes calculated
from the string sigma model for both on- and off-shell boundary interactions.
The boundary state appears as a coherent superposition of closed string states,
and using this observation a proposal for calculating amplitudes beyond tree
level is developed. The application of this technique to more general, time
dependent backgrounds is also discussed.
