  The anomalous frequency and doping dependence of the Hall constant in the
normal state of high-T_c superconductors are investigated within models of
strongly correlated electron systems. In Mori theory, the transition of the
Hall constant from infinite to zero frequency is described by a memory
function. It naturally introduces a second time scale, that, within the t-J
model, is identified with the spinon relaxation time of Anderson. This provides
us with a phenomenological understanding of the interplay between the frequency
and temperature dependence of the Hall constant for frequencies below the
Mott-Hubbard gap. Within the single-band Hubbard model in the limit $U\gg t$,
the memory function is calculated via its moments and shown to project out the
high-energy scale U. This causes the Hall constant to decrease by a factor
$(1+\delta)/2$ ($\delta$: doping), when the frequency is lowered from infinity
to values within the Mott-Hubbard gap. Finally, it is outlined, how the Hall
constant may be calculated in the low frequency regime.
