Hidden Order in $UR{u}_{2}S{i}_2$

We review current attempts to characterize the underlying nature of the hidden order in $UR{u}_{2}S{i}_2$. A wide variety of experiments point to the existence of two order parameters: a large primary order parameter of unknown character which co-exists with secondary antiferromagnetic order. Current theories can be divided into two groups determined by whether or not the primary order parameter breaks time-reversal symmetry. We propose a series of experiments designed to test the time-reversal nature of the underlying primary order in $UR{u}_{2}S{i}_2$ and to characterize its local single-ion physics.