  Field theories are usually quantized by performing a path integral over
configurations of classical fields. This is the case both in perturbation
theory and in Wilson's nonperturbative lattice field theory. D-theory is an
alternative nonperturbative formulation of field theory in which classical
fields emerge from the low-energy collective dynamics of discrete quantum
variables (quantum spins and their gauge analogs -- quantum links) which
undergo dimensional reduction. D-theory was developed some time ago as a
discrete approach to U(1) and SU(2) pure gauge theories, extended to SU(N)
gauge theories and full QCD, and also applied to a variety of other models. On
the practical side, D-theory provides a framework for the development of
efficient numerical methods, such as cluster algorithms. For example, in the
D-theory formulation of CP(N-1) models one can simulate efficiently at non-zero
chemical potential or at non-zero vacuum angle theta. On the conceptual side,
D-theory offers a natural solution for the nonperturbative hierarchy problem of
chiral symmetry in QCD. We also take a broader nonperturbative view on
fundamental physics and speculate that D-theory variables -- i.e. quantum spins
and quantum links -- may be promising candidates for the physical degrees of
freedom that Nature has chosen to regularize the standard model physics at
ultra-short distances.
