  This commentary reflects on the 1930 discoveries of L\'eon Rosenfeld in the
domain of phase-space constraints. We start with a short biography of Rosenfeld
and his motivation for this article in the context of ideas pursued by W.
Pauli, F. Klein, E. Noether. We then comment on Rosenfeld's General Theory
dealing with symmetries and constraints, symmetry generators, conservation laws
and the construction of a Hamiltonian in the case of phase-space constraints.
It is remarkable that he was able to derive expressions for all phase space
symmetry generators without making explicit reference to the generator of time
evolution. In his Applications, Rosenfeld treated the general relativistic
example of Einstein-Maxwell-Dirac theory. We show, that although Rosenfeld
refrained from fully applying his general findings to this example, he could
have obtained the Hamiltonian. Many of Rosenfeld's discoveries were
re-developed or re-discovered by others two decades later, yet as we show there
remain additional firsts that are still not recognized in the community.
