  We discuss the discovery of pressure-induced antiferromagnetism in URu_2Si_2,
in the context of neutron, NMR and \mu SR results. The identification of a
critical pressure separating mean-field and Ising phase transitions leads us to
propose that the system lies close to a bicritical point associated with
magnetic and (non-magnetic) hidden order. We conclude that the recent
observation of an isotropic, field-independent component in the silicon NMR
line-width implies that the hidden order parameter breaks time-reversal
invariance and present a preliminary discussion of the underlying nature of the
hidden order parameter.
