  We prove that the Casimir operator acting on sections of a homogeneous vector
bundle over a generalized flag manifold naturally extends to an invariant
differential operator on arbitrary parabolic geometries. We study some
properties of the resulting invariant operators and compute their action on
various special types of natural bundles. As a first application, we give a
very general construction of splitting operators for parabolic geometries. Then
we discuss the curved Casimir operators on differential forms with values in a
tractor bundle, which nicely relates to the machinery of BGG sequences. This
also gives a nice interpretation of the resolution of a finite dimensional
representation by (spaces of smooth vectors in) principal series
representations provided by a BGG sequence.
