A timetable is a temporal arrangement of a set of meetings such that all given constraints are satisfied. A timetabling problem can be suitably modelled in terms of a set of constraints. We use Constraint Logic Programming and develop methods, techniques and concepts for a combination of interactive and automatic timetabling of university courses and school curricula. An exemplary application of such a timetabling system was developed for the Charité Medical Faculty at the Humboldt University, Berlin. The timetabling system is flexible enough to take into account special user requirements and to allow constraints to be modified easily if no basic conceptual change in the timetabling is necessary. An essential component is an automatic heuristic solution search with an interactive user-intervention facility. The user will, however, only be able to alter a timetable or schedule such that no hard constraints are violated.