A cartridge for an aerosol provision system comprising the cartridge and a control unit, wherein the system comprises a vaporiser for vaporising an aerosolisable material, wherein the cartridge comprises: a mouthpiece outlet; an air channel extending from an air inlet for the cartridge to the mouthpiece outlet via an aerosol generation region; a reservoir for containing an aerosolisable material for aerosolising; an aerosol outlet tube (38;62), extending between the aerosol generating region and the mouthpiece outlet, for directing aerosol generated in the aerosol generating region through to the mouthpiece outlet, wherein the reservoir extends around the aerosol outlet tube (38;62); wherein the cartridge further comprises an aerosolisable-material-level observation means for allowing a user to observe a level of aerosolisable material inside the reservoir; wherein the aerosolisable-material-level observation means comprises a portion of the cartridge, and wherein the portion of the cartridge forms a first wall portion of the reservoir; and wherein a surface of the outlet tube (38;62) is visible from the portion of the cartridge; wherein the cartridge comprises a first surface, a second surface, and a first side surface and a second side surface, wherein the side surfaces (209C;209D) are located between the first surface and the second surface; wherein the first side surface and the second side surface are inclined towards the mouthpiece outlet; and wherein the portion of the cartridge is located on both the side surfaces (209C:209D); wherein the portion of the cartridge further comprises: a first, elongate, window for viewing into the reservoir, wherein the first window is on the first side surface; and a second, elongate, window for viewing into the reservoir, wherein the second window is on the second side surface; wherein each window comprises a maximum dimension of no more than 20mm. a mouthpiece outlet; an air channel extending from an air inlet for the cartridge to the mouthpiece outlet via an aerosol generation region; a reservoir for containing an aerosolisable material for aerosolising; an aerosol outlet tube (38;62), extending between the aerosol generating region and the mouthpiece outlet, for directing aerosol generated in the aerosol generating region through to the mouthpiece outlet, wherein the reservoir extends around the aerosol outlet tube (38;62); wherein the cartridge further comprises an aerosolisable-material-level observation means for allowing a user to observe a level of aerosolisable material inside the reservoir; wherein the aerosolisable-material-level observation means comprises a portion of the cartridge, and wherein the portion of the cartridge forms a first wall portion of the reservoir; and wherein a surface of the outlet tube (38;62) is visible from the portion of the cartridge; wherein the cartridge comprises a first surface, a second surface, and a first side surface and a second side surface, wherein the side surfaces (209C;209D) are located between the first surface and the second surface; wherein the first side surface and the second side surface are inclined towards the mouthpiece outlet; and wherein the portion of the cartridge is located on both the side surfaces (209C:209D); wherein the portion of the cartridge further comprises: a first, elongate, window for viewing into the reservoir, wherein the first window is on the first side surface; and a second, elongate, window for viewing into the reservoir, wherein the second window is on the second side surface; wherein each window comprises a maximum dimension of no more than 20mm. a first, elongate, window for viewing into the reservoir, wherein the first window is on the first side surface; and a second, elongate, window for viewing into the reservoir, wherein the second window is on the second side surface; wherein each window comprises a maximum dimension of no more than 20mm.