  Using the conventional scaling approach as well as the renormalization group
analysis in $d=2+\epsilon$ dimensions, we calculate the localization length
$\xi(B)$ in the presence of a magnetic field $B$. For the quasi 1D case the
results are consistent with a universal increase of $\xi(B)$ by a numerical
factor when the magnetic field is in the range
$\ell\ll{\ell_{\!{_H}}}\alt\xi(0)$, $\ell$ is the mean free path,
${\ell_{\!{_H}}}$ is the magnetic length $\sqrt{\hbar c/eB}$. However, for
$d\ge 2$ where the magnetic field does cause delocalization there is no
universal relation between $\xi(B)$ and $\xi(0)$. The effect of spin-orbit
interaction is briefly considered as well.
