  Recent measurements of the Gottfried sum rule have focused attention on the
possibility of substantial flavor symmetry breaking in the proton sea. This is
confirmed by pp and pD Drell-Yan processes measured at FNAL. Theoretical models
used to `prove' flavor symmetry breaking rely on the assumption of charge
symmetric parton distributions. Substantial charge symmetry violation [CSV]
could affect current tests of flavor symmetry. In this review we examine the
possibility of CSV for parton distributions. We first give definitions for
structure functions without making the usual assumption of charge symmetry.
Next we make estimates of CSV for both valence and sea quark distributions. We
list a set of relations which must hold if charge symmetry is valid, and review
current experimental limits on CSV. We propose a series of experimental tests
of charge symmetry. These tests could either detect CSV, or they could provide
stronger upper limits on CSV effects. We discuss CSV contributions to sum
rules, and we propose sum rules which could differentiate between flavor
symmetry, and charge symmetry violation in nuclear systems.
