  Different decompositions (sum rules) for the proton mass have been proposed
in the literature. All of them are related to the energy-momentum tensor in
quantum chromodynamics. We review and revisit these decompositions by paying
special attention to recent developments with regard to the renormalization of
the energy-momentum tensor. The connection between the sum rules is discussed
as well. We present numerical results for the various terms of the mass
decompositions up to 3 loops in the strong coupling, and consider their scheme
dependence. We also elaborate on the role played by the trace anomaly and the
sigma terms.
