Magnetic properties of diluted magnetic semiconductors and magnetic metallic multilayers are investigated by SQUID magnetometry. By doping GaAs with magnetic Mn2+ ions under well defined growth conditions, one obtains a diluted magnetic semiconductor, (Ga,Mn)As, in which the randomly-distributed magnetic ions act as acceptor centers. At high enough dopant and hole concentration a carrier-induced ferromagnetic state results between the magnetic ions. Due to peculiarities of the growth process sizable amounts of donor defects, such as Mn interstitials and As antisites, are also introduced into the GaAs host. The magnetic properties of (Ga,Mn)As are altered by the presence of such defects through the compensation effect of the holes. The Mn interstitials are thermally unstable above a certain threshold temperature and therefore their concentration can be controlled by post-growth annealing. The influence of the interfaces on the magnetic moment of FeNi/V and FeNi/Co superlattices has been studied. A decrease of the `FeNi' magnetic moment at the interfaces is observed for FeNi/V superlattices while in case of FeNi/Co an enhanced magnetic moment is obtained at the interfaces. Changes of the interlayer exchange coupling have been studied in a series of Fe/V(Fe) multilayers in which the V spacer was alloyed with small amounts of Fe. The dynamic magnetic properties of discontinuous metal-insulator multilayers of Ni81Fe19/Al2O3 have been investigated. By varying the thickness of the insulator the system exhibits a superferromagnetic, a 3d spin-glass-like and a superparamagnetic behavior.