  We study F-theory duals of heterotic line bundle models on elliptically
fibered Calabi-Yau threefolds. These models necessarily contain NS5-branes
which are geometrised in the dual F-theory compactifications. We initiate a
systematic study of the correspondence between various configurations of
NS5-branes and the dual geometries in F-theory and perform several checks of
the duality. Furthermore, we discuss the singular transitions between different
configurations of NS5-branes.
