  D-theory is an alternative non-perturbative approach to quantum field theory
formulated in terms of discrete quantized variables instead of classical
fields. Classical scalar fields are replaced by generalized 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 scalar field
theories with various global symmetries and of gauge theories with various
gauge groups is constructed explicitly and the mechanism of dimensional
reduction is investigated.
