  A unified field theory based on the compactification of a higher
D-dimensional Einstein-Yang-Mills-Higgs action is developed. The extra D-4
dimensions form a compact internal space with scale size R. An anomaly-free
unified chiral model of quarks and leptons, described by SO(18) in twelve
dimensions, breaks down to $SO(10)\times SO(8)\to SO(10)$ with a non-trivial
topological structure and three chiral families in four dimensions. A quantum
field theory formalism in D-dimensions leads to a self-consistent, finite
quantum gravity, Yang-Mills and Higgs theory, which is unitary and gauge
invariant to all orders of perturbation theory. The gauge hierarchy problem is
solved due to the exponential damping of the Higgs self-energy loop graph for
energies greater than $\sim 1$ TeV, and because of the reduction of quantum
gravity to a scale of several TeV. The compactification scale is $M_c\geq 1$
TeV, leading to Kaluza-Klein excitations and experimental signatures at a scale
of several TeV. Various scenarios for evading fast proton decay are discussed.
