Anisotropic scaling in depinning of a flux line.

We study the depinning of a flux line by analytical and numerical methods applied to a phenomenological equation of motion. Transverse fluctuations do not influence the critical behavior of the longitudinal component, justifying ``planar approximations". In an isotropic medium, longitudinal fluctuations have a roughness exponent $\zeta_\parallel=1$, and relax with a dynamic exponent $z_\parallel\approx4/3$; transverse fluctuations are suppressed ($\zeta_\perp=1/2<\zeta_\parallel$), and relax more slowly, with $z_\perp=z_\parallel+1$. Anisotropy in the depinning threshold, or orientational dependence of force-force correlations, lead to new universality classes.