This chapter summarizes the theory of micromagnetics—a theory by far having the largest utility to understand magnetic phenomena in ferromagnetic materials. It is intended to be a crash course—an attempt to cover all the background information before one can try their hand with one of many available simulation tools. Micromagnetic models faithfully capture the dynamics and the nonlinear behaviors of magnetic materials all the way down to nanometer size scales, i.e., practically all dimensions that are relevant to device applications. In this chapter the focus will be on domain theory and the switching and hysteresis behavior of small particle, while magnetic excitations will be discussed in Chap. 3 .