This is a book trying to give a systematic theory of computational applied magnetism, based on Maxwell equations of fields and Landau-Lifshitz equations of magnetic moments. The focused magnetic materials are magnetic recording materials utilized in computer hard disk drives. Traditionally, “Micromagnetics” includes the areas of “magnetization curve theory", “domain theory" and “read and write process analyses in recording systems”. As Springer Briefs, this book includes the first two areas of micromagnetics. M-H loops of hard magnetic thin film media, soft magnetic layers and tunneling magneto resistive spin valves are solved based on the microstructures of thin films.