A vapour provision system comprising: 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; wherein the optical detector is located to detect light emitted by the optical source that has been reflected into an indirect optical path from a surface of aerosolisable substrate material in the storage volume, and the optical detector is configured to spatially resolve a position at which light emitted from the optical source is detected; and the optical source is arranged to emit light to be incident on the surface of the aerosolisable substrate material at a non-perpendicular angle such that the reflected light is incident on the optical detector at a position that varies with the depth of the aerosolisable substrate material; wherein the characteristic of the light is the detected position on the optical detector, and the controller is configured to deduce the depth of the aerosolisable substrate material from the detected position; and in which the vapour provision system comprises a cartomiser and a control unit separably connectable to one another, wherein the reservoir is comprised in the cartomiser portion, and the optical detector, the optical source and the controller are comprised in the control unit. 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; wherein the optical detector is located to detect light emitted by the optical source that has been reflected into an indirect optical path from a surface of aerosolisable substrate material in the storage volume, and the optical detector is configured to spatially resolve a position at which light emitted from the optical source is detected; and the optical source is arranged to emit light to be incident on the surface of the aerosolisable substrate material at a non-perpendicular angle such that the reflected light is incident on the optical detector at a position that varies with the depth of the aerosolisable substrate material; wherein the characteristic of the light is the detected position on the optical detector, and the controller is configured to deduce the depth of the aerosolisable substrate material from the detected position; and in which the vapour provision system comprises a cartomiser and a control unit separably connectable to one another, wherein the reservoir is comprised in the cartomiser portion, and the optical detector, the optical source and the controller are comprised in the control unit. 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; wherein the optical detector is located to detect light emitted by the optical source that has been reflected into an indirect optical path from a surface of aerosolisable substrate material in the storage volume, and the optical detector is configured to spatially resolve a position at which light emitted from the optical source is detected; and the optical source is arranged to emit light to be incident on the surface of the aerosolisable substrate material at a non-perpendicular angle such that the reflected light is incident on the optical detector at a position that varies with the depth of the aerosolisable substrate material; wherein the characteristic of the light is the detected position on the optical detector, and the controller is configured to deduce the depth of the aerosolisable substrate material from the detected position; and in which the vapour provision system comprises a cartomiser and a control unit separably connectable to one another, wherein the reservoir is comprised in the cartomiser portion, and the optical detector, the optical source and the controller are comprised in the control unit.