Ferromagnetic resonance (FMR) measurements probe the response of magnetic systems within the nanosecond-regime due to an excitation within the microwave regime. Due to the high sensitivity of FMR this technique is well suited for the investigation of nanostructures and ultrathin magnetic films or multilayers. As the resonance condition is determined by internal fields like anisotropy fields or interlayer coupling fields within layered structures, FMR experiments give direct and quantitative access to these quantities based on an analysis that uses the Landau-Lifshitz equation of motion. This will be demonstrated for the case of Ni-Cu-Ni films grown epitaxially on Cu(100) substrates and for highly monodisperse Co-CoO core-shell particles of about 10 nm diam. In case of the films the unique possibility to grow and measure the samples within an ultrahigh vacuum environment is presented. (c) 2005 American Vacuum Society.