  We present a general prescription for determining the global U(1) symmetries
of six-dimensional superconformal field theories (6D SCFTs). We use the
quiver-like gauge theory description of the tensor branch to identify candidate
U(1) symmetries which can act on generalized matter. The condition that these
candidate U(1)'s are free of Adler-Bell-Jackiw (ABJ) anomalies provides
bottom-up constraints for U(1)'s. This agrees with the answer obtained from
symmetry breaking patterns induced by Higgs branch flows. We provide numerous
examples illustrating the details of this proposal. In the F-theory realization
of these theories, some of these symmetries originate from deformations of
non-abelian flavor symmetries localized on a component of the discriminant,
while others come from an additional generator of the Mordell-Weil group. We
also provide evidence that some of these global U(1)'s do not arise from gauge
symmetries, as would happen in taking a decoupling limit of a model coupled to
six-dimensional supergravity.
