  After recalling a few basic concepts from cosmology and string theory, I will
discuss the main ideas/assumptions underlying string cosmology and show how
these lead to a two-parameter family of ``minimal" models. I will then explain
how to compute, in terms of those parameters, the spectrum of scalar, tensor
and electromagnetic perturbations, point at their ($T$ and $S$-type) duality
symmetries, and mention their most relevant physical consequences.
