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.