  We show that infinitesimal automorphisms and infinitesimal deformations of
parabolic geometries can be nicely described in terms of the twisted de-Rham
sequence associated to a certain linear connection on the adjoint tractor
bundle. For regular normal geometries, this description can be related to the
underlying geometric structure using the machinery of BGG sequences. In the
locally flat case, this leads to a deformation complex, which generalizes the
is well know complex for locally conformally flat manifolds.
  Recently, a theory of subcomplexes in BGG sequences has been developed. This
applies to certain types of torsion free parabolic geometries including,
quaternionic structures, quaternionic contact structures and CR structures. We
show that for these structures one of the subcomplexes in the adjoint BGG
sequence leads (even in the curved case) to a complex governing deformations in
the subcategory of torsion free geometries. For quaternionic structures, this
deformation complex is elliptic.
