Nuclear reactions induced by high energy radioactive beams create a transient state of nuclear matter with high density and appreciable neutron to proton asymmetry. This will provide a unique opportunity to explore novel properties of dense neutron-rich matter and the isospin-dependence of the nuclear equation of state (EOS). Here we study the $\pi^-/\pi^+$ ratio as a probe of the EOS of dense neutron-rich matter.
