Magneto-optical imaging (MOI) based on the large Faraday effect in Bi:YIG films is a unique method allowing real-time visualization of dynamic magnetic phenomena in materials, in particular superconductors. In the first part, I describe briefly how we synthesize the garnet sensor films, and how MOI experiments are carried out at cryogenic temperatures. Some years ago, our group succeeded to bring MOI to a level where individual Abrikosov vortices (magnetic flux quanta) could be observed, and their motion directly monitored. I will report on this development, and illustrate by VIDEO clips how MOI can reveal the detailed dynamical behavior of vortex matter. On a larger scale, we have used MOI to discover that in films of many superconductors, the penetration of flux often occurs in abrupt bursts of (beautiful!) dendritic flux structures of dimension up the full sample size. Avalanches in films of MgB2, YBaCuO, Nb and NbN will be presented to show the various aspects of this dramatic phenomenon. Finally, the origin of these avalanche events, which are very harmful for practical applications, will be discussed.