The properties of a thin film which cause its magnetic behaviour to differ from that of bulk material are first explained. The hysteresis loops are shown to be characteristic of a uniaxial magnetic anisotropy, and the origin of that anisotropy is briefly discussed. Deviations from the theoretically ideal behaviour of a uniaxial film are explained in terms of inhomogeneity of film structure. Some of the different kinds of domain walls observed in films are described and illustrated. The thickness dependence of the saturation magnetization is shown to be relevant to the theory of ferromagnetism in metals, and resonance phenomena are discussed in the same light. Finally the preparation of magnetic films and their practical applications are considered.