  We consider both "bottom-up" and "top-down" approaches to the origin of gauge
kinetic mixing. We focus on the possibilities for obtaining kinetic mixings
$\epsilon$ which are consistent with experimental constraints and are much
smaller than the naive estimates ($\epsilon \sim 10^{-2} - 10^{-1}$) at the
one-loop level. In the bottom-up approach, we consider the possible suppression
from multi-loop processes. Indeed we argue that kinetic mixing through gravity
alone, requires at least six loops and could be as large as $\sim 10^{-13}$. In
the top-down approach we consider embedding the Standard Model and a $U(1)_X$
in a single grand-unified gauge group as well as the mixing between Abelian and
non-Abelian gauge sectors.
