In this expository article we discuss the relations between Sasakian
geometry, reduced holonomy and supersymmetry. It is well known that the
Riemannian manifolds other than the round spheres that admit real Killing
spinors are precisely Sasaki-Einstein manifolds, 7-manifolds with a nearly
parallel G2 structure, and nearly Kaehler 6-manifolds. We then discuss the
relations between the latter two and Sasaki-Einstein geometry.Comment: 40 pages, some minor corrections made, to appear in the Handbook of
pseudo-Riemannian Geometry and Supersymmetr