Ever since the turn of the century sky patrol observations—i.e., the routine surveillance of the visible night sky—has been one of the main foundations of variable star research, and thus of astronomy in general. Historically several of the major Observatories all around the world were contributing. Up to this day all these observations are in the form of large-field photographic plates, constituting a total of some 2 million plates which, collectively, contain the history of the light changes of celestial objects (mostly in the northern hemisphere and down to some 13 magnitudes apparent brightness or fainter) during the past 100 years. In modern times the last place left in the world where sky patrols are still being carried out routinely is Sonneberg Observatory.