In this introductory talk we present basic concepts of micromagnetics - a phenomenological theory developed to study magnetization configurations in ferromagnets on a mesoscopic (from ~10 nm to ~10 mkm) length scale. Our main focus is on micromagnetic numerical simulations, which are widely used to study a large variety of magnetic systems - from disordered assemblies of single-domain particles to extended thin films and patterned nanoelement arrays. In the talk we discuss both simulations of equilibrium (static) magnetization configurations and simulations of the magnetization dynamics, which allow to explore time evolution of the magnetization in a sub-nanosecond to microsecond time range.