A chair comprises a seat portion for enabling a user to sit on the chair and support means for supporting the seat portion. The seat portion is movable between a plurality of different height positions on the support means, and includes locking means adapted for engagement with and disengagement from the support means at each height position, by rocking the seat portion relative to the support means, wherein the seat portion is locked relative to the support means when the locking means is in the engaged position and free to move relative to the support means between different height positions when the locking means is disengaged.