This chapter is summarizing the basic concepts of the micromagnetic theory extensively used for modeling ferromagnetic nanostructured systems, including thin films, wires, and nanopillars. In the first section, the assumptions used in micromagnetism are listed and the ingredients which are necessary for establishing the general equations governing the magnetic stable state and the magnetization dynamic (interactions, energies, fields) are introduced. The second section illustrates how micromagnetic theory is used for describing the magnetic behavior of nanosize magnetic samples, in particular the difference between multidomain single domain states. The third part shows the difference between the macrospin and micromagnetic models in the study of spin-transfer torque magnetic random-access memory (STT-MRAM) with in-plane magnetized layers. The role of interlayer dipolar coupling in the magnetization reversal process in STT-MRAM is discussed in the last part.