We discuss the theory of domain wall pinning in bulk ferromagnetic materials as applied to the coercive force. A new theory which includes the wave nature of the ferromagnetic domain walls is explained. Specifically, an explicit algebraic expression for the magnetic field required to move a 180° ferromagnetic domain wall across a planar defect such as a grain boundary is obtained. The results show that the coercive force is proportional, amongst other factors, to the ratio of the barrier width to the domain wall width and to the fractional changes in the exchange and anisotropy constants upon enetering the defect region. Using this expression, one can obtain the correct order of magnitude for many of the low and high coercive materials whose magnetic properties are subject to the mechanism of domain wall motion. Applications to practical physical problems are suggested.