The last two chapters have described the growth and structure of epitaxial films. Here, the basic aspects of ultrathin-film magnetism and their relationship to structure and growth will be the focus. First, theoretical models to describe the ferromagnetism and antiferromagnetism of 3d metals are introduced in Sect. 4.1. In addition, the relationship between structure and magnetism for bulk phases is discussed. In Sect. 4.2, magnetic properties of surfaces and thin films receive attention. In particular, the correlation between structure and magnetism will be of key interest. The magnetic anisotropy, which is important for technological applications, is also described in Sect. 4.2. Finally, our attention is turned to the most commonly used experimental techniques for the determination of magnetic properties of ultrathin films. These techniques cover the surface magnetooptic Kerr effect, ferromagnetic resonance measurements, and x-ray magnetic dichroism in both the edge absorption mode and the photoemission mode.