  We classify (1+3)-dimensional Schr\"odinger equations for a particle
interacting with the electromagnetic field that are solvable by the method of
separation of variables. As a result, we get eleven classes of the
electromagnetic vector potentials of the electromagnetic field $A(t, \vec
x)=(A_0(t, \vec x)$, $\vec A(t, \vec x))$ providing separability of the
corresponding Schr\"odinger equations. It is established, in particular, that
the necessary condition for the Schr\"odinger equation to be separable is that
the magnetic field must be independent of the spatial variables. Next, we prove
that any Schr\"odinger equation admitting variable separation into second-order
ordinary differential equations can be reduced to one of the eleven separable
Schr\"odinger equations mentioned above and carry out variable separation in
the latter. Furthermore, we apply the results obtained for separating variables
in the Hamilton-Jacobi equation.
