Two-Time Physics

We give an overview of the correspondance between one-time-physics and two-time-physics. This is characterized by the presence of an SO(d,2) symmetry and an Sp(2) duality among diverse one-time-physics systems all of which can be lifted to the same more symmetric two-time-physics system by the addition of gauge degrees of freedom. We provide several explicit examples of physical systems that support this correspondance. The example of a particle moving in (AdS_D) X (S^n), with SO(D+n,2) symmetry which is larger than the popularly known symmetry SO(D-1,2) X SO(n+1) for this case, should be of special current interest in view of the proposed AdS-CFT duality.