Hidden order in URu2Si2

We review current attempts to characterize the underlying nature of the hid den order in URu2Si2. A wide variety of experiments point to the existence of two order parameters: a large primary order parameter of unknown charact er which co-exists with secondary antiferromagnetic order. Current theories can be divided into two groups determined by whether or not the primary or der parameter breaks time-reversal symmetry. We propose a series of experim ents designed to test the time-reversal nature of the underlying primary or der in URu2Si2 and to characterize its local single-ion physics.