A survey paper that details what was known at the time about Riemannian metrics with reduced holonomy.
This paper has since been superceded by S. Salamon's 1989 book on Riemannian geometry and holonomy. However both of these are now inadequate. The results of D. Joyce show that there exist compact manifolds with holonomy G2 or Spin(7). Considerably more is now known about the geometry of the so-called quaternionic-Kähler manifolds. The works of Joyce, Salamon, and LeBrun should be consulted for further information on these subjects.