This book addresses aspects of neutron scattering associated with magnetic systems in disorder. These systems range in complexity from an isolated defect in a ferromagnet to materials such as amorphous spin glass materials which have massive disorder in terms of both atomic arrangement and magnetism. The primary focus of the text is neutron scattering but the author draws a connection with other probes of magnetic systems whenever appropriate. Topics covered include: the relationship between the neutron cross section and the magnetization and susceptibility of magnetic materials; polarized neutron techniques; and properties of dilute ferromagnetic alloys, spin glasses, and amorphous magnets.