  Over the last few years, string theory has changed profoundly. Most
importantly, novel duality relations have emerged which involve gauge theories
of brane excitations on one side and various closed string backgrounds on the
other. In this lecture, we introduce the fundamental ingredients of modern
string theory and explain how they are modeled through 2D (boundary) conformal
field theory. This so-called `microscopic description' of strings and branes is
an active research area with new results ranging from the classification and
construction of boundary conditions to studies of 2D renormalization group
flows. We shall provide an overview of such developments before concluding the
lecture with an extensive outlook on some research that is motivated by current
problems in string theory. This includes investigations of non-rational and
non-unitary conformal field theories.
