An adjustable helmet having first and second shells and a single actuator comprising at least one tooth, the single actuator being moveable between: a first locked position, wherein the at least one tooth retains the first and second shells and the first and second shells define a first hollow space; a released position, wherein the at least one tooth no longer retains the first and second shells such that one of the first and second shells is moveable relative to the other of the first and second shells; and a second locked position, wherein the at least one tooth retains the first and second shells and the first and second shells define a second hollow space, the second hollow space being different than the first hollow space.