  A set of Grand Unified Theories based upon the gauge groups $SU(5)_\L \times
SU(5)_\R$, $SO(10)_\L \times SO(10)_\R$ and $SU(4)_\C \times SU(4)_\L \times
SU(4)_\R$ is explored. Several novel features distinguish these theories from
the well-known $SU(5)$, $SO(10)$ and $SU(4)_\C \times SU(2)_\L \times SU(2)_\R$
models which they generalize. Firstly, Standard Model quarks and leptons are
accompanied by and mix with heavy $SU(2)_\L \times SU(2)_\R$ singlet partners.
The resulting fermion mass matrices are seesaw in form. Discrete parity
symmetries render the determinants of these mass matrices real and eliminate CP
violating gauge terms. The unified seesaw models consequently provide a
possible resolution to the strong CP problem. Secondly, $\sinsq$ at the
unification scale is numerically smaller than the experimentally measured $Z$
scale value. The weak angle must therefore increase as it evolves down in
energy. Finally, proton decay is suppressed by small seesaw mixing factors in
all these theories.
