In Section 1.1 it is argued that ferromagnetic materials with a high resistivity are of considerable practical importance. In Section 1.2 the electric and magnetic properties of ferrite materials at various frequencies are discussed. In Section 1.3 an account is given of the physicochemical background of the properties of ferrites. In Section 1.4 the manufacture is dealt with and the important possibilities for application of different ferrite materials are briefly reviewed. In the following Sections 2.1–2.5 the experimental methods are described which are used for themeasurements of the electric and magnetic properties of the materials. Section 2.2 deals with a bridge used at frequencies between 1 and 100 kc/s. Sections 2.3 and 2.4 contain a description of themethods used for measurements at frequencies up to 500 Mc/s, while Section 2.5 deals with experiments in the microwave region.