  Columnar defects provide effective pinning centers for magnetic flux lines in
high--$T_{\rm c}$ superconductors. Utilizing a mapping of the statistical
mechanics of directed lines to the quantum mechanics of two--dimensional
bosons, one expects an entangled flux liquid phase at high temperatures,
separated by a second--order localization transition from a low--temperature
``Bose glass'' phase with infinite tilt modulus. Recent decoration experiments
have demonstrated that below the matching field the repulsive forces between
the vortices may be sufficiently large to produce strong spatial correlations
in the Bose glass. This is confirmed by numerical simulations, and a remarkably
wide soft ``Coulomb gap'' at the chemical potential is found in the
distribution of pinning energies. At low currents, the dominant transport
mechanism in the Bose glass phase proceeds via the formation of double kinks
between not necessarily adjacent columnar pins, similar to variable--range
hopping in disordered semiconductors. The strong correlation effects
originating in the long--range vortex interactions drastically reduce
variable--range hopping transport.
