The absolute Galois group of a number field is a mysterious
object, that one can try to understand by means of its representations. It
is known that holomorphic cuspidal modular forms are, in a suitable sense,
a source of many p-adic Galois representations. More generally, it is
conjectured that also torsion classes in the coherent cohomology of
Shimura varieties have attached Galois representations, with prescribed
local properties. I will give an introduction to these themes, and present
results obtained in collaboration with Matthew Emerton and Liang Xiao
toward the conjecture.