A vapour provision system comprises: a reservoir defining a storage volume to hold aerosolisable substrate material； and an aerosolisable substrate material detection system comprising: an optical source located externally of the reservoir and operable to emit light into the storage volume； an optical detector located externally of the reservoir and operable to detect light emitted by the optical source that has traversed an optical path through the storage volume； and a controller configured to determine a characteristic of light detected by the optical detector and deduce information regarding a depth of aerosolisable substrate material in the storage volume from the determined characteristic of the light.
