It is proposed that magnetization reversal in polycrystalline ferro‐ and ferrimagnetic materials is primarily due to the nucleation and growth of 180° Bloch walls. The origin of domains of reverse magnetization is discussed. The rate of growth of these domains is determined by a study of the elastic and frictional forces which retard the motion of their 180°‐Bloch‐wall boundaries. This theoretical model successfully explains the output‐voltage wave forms of polycrystalline materials. A figure of merit for the magnetization reversal of magnetic cores is defined as the switching coefficient Sw=(Hm-H0)τ, where τ is the time required to reverse the magnetization, Hm is the applied magnetic field, and H0 is the threshold field value at which the average domain‐wall velocity is zero. Sw is composed of an eddy‐current contribution Swe and a spin‐relaxation contribution Swr. The value of Sw is derived in terms of various fundamental parameters of the material. It is shown that in ferrites and ultra‐thin metal tapes, Swe«Swr. Theoretical relationships expressing the contributions of spin‐relaxation, eddy‐current, and hysteresis effects to energy losses are derived on the basis of this model. A study of the factors which affect the magnetization‐reversal time of materials with square hysteresis loops indicates that a better figure of merit will result if the proper hysteresis shape is obtained by grain alignment rather than by the techniques currently employed in ferrites. A number of experiments are presented in support of this model.