A guide for a surgical instrument has a support and an instrument holder. One of the support and the instrument holder has a partial spherical surface, and the other has three of more contact points arranged to be coincident with the surface of a sphere having substantially the same radius as the spherical surface. In use, the spherical surface and the contact points are placed in contact with each other, and the orientation of a surgical instrument can be adjusted by relative movement of the contact points and the spherical surface.
