A plastics material mold for casting a contact lens from curable material. Disposable male and female mold members (20,22;50,51) define a mold cavity and each having a curved surface (21,23;52,54) for molding respective optical surface of the contact lens, the male mold member having a shoulder (26,53) which is a slidable fit with a generally cylindrical, e.g. frusto-conical, surface (25,55) on a female mold member to permit relative mold member movement during curing without substantially affecting the optical quality of the molded optical surfaces. The sliding fit shoulder and female surface are relatively simple to mold with accuracy and thus permit the edge (29) of the eventual cast lens to be formed relatively reliably and accurately.