We investigate spin waves - the eigenmodes of ferromagnets - in thin films by time resolved Kerr microscopy. With this technique we are able to directly image spin waves and obtain their characteristics such as wavelength and attenuation length (or simply the complex wave vector). Spin waves are very sensitive tools to probe the magnetic properties of their media – magnetic thin films. As examples, we investigate their response to (i) the application of spin-polarized currents (in so called Spin Wave Doppler experiments), (ii) the modification of interfaces or the composition of the thin films and (iii) the confinement in magnetic nanostructures. Spin waves are therefore a nice probe to study magnetization dynamics in great detail.