  A new non-perturbative approach to quantum field theory is proposed. Instead
of performing a path integral over configurations of classical fields, D-theory
works with discrete quantized variables. Classical spin fields are replaced by
quantum spins, and classical gauge fields are replaced by quantum links. The
classical fields of a d-dimensional quantum field theory reappear as low-energy
effective degrees of freedom of the discrete variables, provided the
(d+1)-dimensional D-theory is massless. When the extent of the extra Euclidean
dimension becomes small in units of the correlation length, an ordinary
d-dimensional quantum field theory emerges by dimensional reduction. The
D-theory formulation of some spin models and gauge theories is constructed
explicitly. In particular, QCD emerges as a quantum link model.
