Located in the French Alps at the northeastern end of the Chartreuse massif in te nature reserve of Hauts de Chartreuse, the Mont Granier is a half perched syncline of alternating strata of limestone and marls, eastward tilted and very karstified. This mountain, overhanging the valley and the city of Chambéry (Savoie), is famous since the middle age because of the 1248 major landslide that affected its north face, which is known as one of the major historical collapse events in Europe. The Mont Granier instability continues nowadays, with several large rokfalls observed during the 20th Century, and a recent impressive sequence of collapsew and debris-flows during the winter and spring 2016. Thses last events induce a renewed interest for the study and the monitoring of this iconic mountain in order both to prevent risks and to increase scientific knowledge on mass-movement processes. This paper sumarizes the main events and modalities of the Mont Granier rock slope instability and present the collaborative and participatory monitoring device set up after the 2016 instability crisis.