We introduce the classical results of de Rham and Berger on the holonomy of a Riemannian manifold. We compare these to the situation of parallel skew-torsion, where we obtain Riemannian submersions from reducible holonomy. *If time permits I will give an introduction to 3-$(\alpha, \delta)$-Sasaki manifolds and their submersion onto quaternionic Kähler manifolds.