The chapter provides a detailed introduction to magnetic anisotropy of ferromagnetic ultrathin films and its analysis by ferromagnetic resonance on a tutorial level. While the microscopic origins of the magnetic anisotropy as well as recent developments in its theoretical description are shortly discussed, emphasis is put on a phenomenological description using the free energy of the system together with its symmetries. The formalism is used to describe ferromagnetic resonance experiments which present an extremely sensitive method to experimentally investigate magnetic anisotropy in thin film heterostructures. Expressions for the free energy and the resonance equations are derived for the most widely used crystal symmetries such as cubic, tetragonal and hexagonal. The general equations are illustrated by giving selected examples of current research on thin metallic films on different kinds of substrates (MgO, GaAs and Cu).