  We develop a categorical analogue of Clifford theory for strongly graded
rings over graded fusion categories. We describe module categories over a
fusion category graded by a group $G$ as induced from module categories over
fusion subcategories associated with the subgroups of $G$. We define invariant
$\C_e$-module categories and extensions of $\C_e$-module categories. The
construction of module categories over $\C$ is reduced to determine invariant
module categories for subgroups of $G$ and the indecomposable extensions of
this modules categories. We associate a $G$-crossed product fusion category to
each $G$-invariant $\C_e$-module category and give a criterion for a graded
fusion category to be a group-theoretical fusion category. We give necessary
and sufficient conditions for an indecomposable module category to be extended.
