The paper surveys the properties and applications of present-day ferrites intended for use as cores for high-quality inductors and transformers in carrier telephony and similar equipment. In considering the properties, emphasis is placed on the significance of the various parameters used to define ferrite performance. Typical values of these properties are tabulated.Considering the application of ferrites to high-quality inductors, the basic requirements are stated. The realization of these requirements is then discussed, starting with an analysis of the effect of the geometric proportions of the core on its electrical performance and continuing with the design of adjusting mechanisms and the means of assembly. Emphasis is placed on the attainment of the maximum stability of inductance. A similar treatment is given for the application to broadband transformers. It is shown that for bandwidths extending up to 10Mc/s the pot-core shape is suitable, while for bandwidths up to several hundred megacycles per second small, specially designed bead-like cores give very good performance.The paper concludes with a number of graphs showing the performance of typical inductors and transformers using the ferrite cores described.