  The vortex lattice is an ideal system to study the competition and interplay
between interaction and disorder. New results on 2H-NbSe_2 (in samples of
progressively increasing pinning) elucidate how pinning alters the phase
boundary separating the "ordered" and "disordered" states of the flux lines and
causes a novel disorder-induced transition. A generic magnetic phase diagram of
a type II superconductor.
