In vivo microscopy of dynamic processes in cells and organisms requires very fast and sensitive acquisition methods. Confocal laser scanning microscopy is inherently speed-limited by the requirement of beam scanning movements. In contrast to single beam scanning systems, the parallelized approach of multi-beam scanning is much faster. Spinning disk confocal microscopes are therefore very suited for fast in vivo imaging. The principles of spinning disk microscopy will be explained in this chapter and a thorough comparison of the performance of single beam and multi-beam scanning systems is made and illustrated with an example of in vivo imaging in Dictyostelium discoideum.