Two-time physics (2T) is a general reformulation of one-time physics (IT) t
hat displays previously unnoticed hidden symmetries in IT dynamical systems
and establishes previously unknown duality,type relations among them. This
may play a role in displaying the symmetries and constructing the dynamics
of little understood systems, such as M-theory. 2T-physics describes vario
us IT dynamical systems as different d-dimensional 'holographic' views of t
he same 2T system in d + 2 dimensions. The 'holography' is due to gauge sym
metries that tend to reduce the number of effective dimensions. Different I
T evolutions (i.e. different Hamiltonians) emerge from the same 2T-theory w
hen gauge fixing is done with different embeddings of d dimensions inside d
+ 2 dimensions. Thus, in the 2T setting, the distinguished IT which we cal
l 'time' is a gauge-dependent concept. The 2T-action also has a, global SO
(d, 2) symmetry in flat spacetime, or a more general d + 2 symmetry in curv
ed spacetime, under which all dimensions are on an equal footing. This symm
etry is observable in many IT-systems, but it remained unknown until discov
ered in the 2T formalism. The symmetry takes various nonlinear (hidden) for
ms in the IT-systems, and it is realized in the same irreducible unitary re
presentation (the same Casimir eigenvalues) in their quantum Hilbert spaces
. 2T-physics has mainly been developed in the context of particles, includi
ng spin and supersymmetry, but some advances have also been made with strin
gs and p-branes, and insights for M-theory have already emerged. In the cas
e of particles, there exists a general worldline formulation with backgroun
d fields, as well as a field theory formulation, both described in terms of
fields that depend on d + 2 coordinates. All IT particle interactions with
Yang-Mills, gravitational and other fields are included in the d + 2 refor
mulation. In particular, the standard model of particle physics can be rega
rded as a gauge-fixed form of a 2T-theory in 4 + 2 dimensions. These facts
already provide evidence for a new type of higher-dimensional unification.