  `Old' String Theory is a theory of one-dimensional extended objects, whose
vibrations correspond to excitations of various target-space field modes
including gravity. In these lectures I will give an introduction to low-energy
Effective Target-Space Actions derived from conformal invariance conditions of
the underlying sigma models in string theory, discuss cosmology, emphasizing
the role of the dilaton field in inducing inflationary scenaria and in general
expanding string universes. Specifically, I shall analyse some exact solutions
of string theory with a linear dilaton, and discuss their role in inducing
expanding Robertson-Walker Universes. I will mention briefly pre-Big-Bang
scenaria of String Cosmology, in which the dilaton plays a crucial role. In
view of recent claims on experimental evidence on the existence of cosmic
acceleration in the universe today, with a positive non-zero cosmological
constant (de Sitter type), I shall also discuss difficulties of incorporating
such Universes with eternal acceleration in the context of critical string
theory, and present scenaria for a graceful exit from such a phase.
