  We investigate duality properties of N-form fields, provide a symmetric way
of coupling them to electric/magnetic sources, and check that these charges
obey the appropriate quantization requirements. First, we contrast the D=4k
case, in which duality is a well-defined SO(2) rotation generated by a
Chern-Simons form leaving the action invariant, and D=4k+2 where the
corresponding ostensibly SO(1,1) rotation is not only not an invariance but
does not even have a generator. When charged sources are included we show
explicitly in the Maxwell case how the usual Dirac quantization arises in a
fully symmetric approach attaching strings to both types of charges. Finally,
for D=4k+2 systems, we show how charges can be introduced for self-dual
(2k)-forms, and obtain the D=4k models with sources by dimensional reduction,
tracing their duality invariance to a partial invariance in the higher
dimensions.
