An electronic vapour provision system comprising: a housing; a vapouriser contained within the housing; a mouthpiece at one end of said system, said mouthpiece providing an air outlet; a body portion within said housing, said body including at least a power source for the vapouriser; a vapouriser portion within said housing, said vaporiser portion including the vapouriser; one or more air inlet holes provided in a portion of the housing that includes said body portion, whereby in response to a user inhalation at the mouthpiece, air flows into the system through said one or more air inlet holes, past the vapouriser, and out through the mouthpiece; a collar located in the body portion around the portion of the housing in which the one or more air inlet holes are provided, said collar being movable by a user with respect to the housing; and a mechanism located in the body portion for positively engaging the collar and the housing at a plurality of predetermined positions as the collar is moved with respect to the housing, wherein different ones of said plurality of predetermined positions result in different degrees of alignment between collar and the one or more air inlet holes of the housing, thereby providing different levels of ventilation into the system; wherein the electronic vapour provision system has a first state in which the body portion is detached from the vapouriser portion, and a second state in which the body portion has a rigid connection to the vapouriser portion, such that in the second state, movement of the body portion relative to the vapouriser portion is prevented other than to detach the vapouriser portion from the body portion into the first state. a housing; a vapouriser contained within the housing; a mouthpiece at one end of said system, said mouthpiece providing an air outlet; a body portion within said housing, said body including at least a power source for the vapouriser; a vapouriser portion within said housing, said vaporiser portion including the vapouriser; one or more air inlet holes provided in a portion of the housing that includes said body portion, whereby in response to a user inhalation at the mouthpiece, air flows into the system through said one or more air inlet holes, past the vapouriser, and out through the mouthpiece; a collar located in the body portion around the portion of the housing in which the one or more air inlet holes are provided, said collar being movable by a user with respect to the housing; and a mechanism located in the body portion for positively engaging the collar and the housing at a plurality of predetermined positions as the collar is moved with respect to the housing, wherein different ones of said plurality of predetermined positions result in different degrees of alignment between collar and the one or more air inlet holes of the housing, thereby providing different levels of ventilation into the system; wherein the electronic vapour provision system has a first state in which the body portion is detached from the vapouriser portion, and a second state in which the body portion has a rigid connection to the vapouriser portion, such that in the second state, movement of the body portion relative to the vapouriser portion is prevented other than to detach the vapouriser portion from the body portion into the first state.