Pressure-Induced Magnetism and Hidden Order in URu₂Si₂

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.