A panel structure for a demountable partition system usable for dividing a room or for forming rooms and/or corridors or other structures, the panel structure having two opposite major faces (14, 15) and, in use, being positioned in a generally upright orientation with other like panel structures of the demountable partition system, in which there are provided one or more rolling elements (27, 28) movable between an extended, ground-contacting position, and a retracted position within the panel structure, characterised in that: the panel structure comprises a base portion and a panel portion carried on the base portion, the one or more rolling elements (27, 28) are movable between said extended, ground-contacting position and said retracted position within the panel structure using a gearbox mechanism (29, 30), said one or more rolling elements (27, 28) and said gearbox mechanism (29, 30) are self-contained within the base portion, whereby the one or more rolling elements (27, 28) and the gearbox mechanism (29, 30) do not intrude into the panel portion to thereby maintain the integrity of and preserve an operating characteristic of the panel portion, there is provided a reinforced substantially bullet-proof layer between the said two opposite major faces (14, 15) of the panel structure, the said reinforced substantially bullet-proof layer lies in a plane inclined to the family of planes defined by the said two opposite major faces (14, 15) of the panel structure, the panel structure being defined along at least two opposite sides by edges having half lap tongues (25, 26) which overlap one another when two like panel structures are placed in juxtaposition with respect to one another, the said half lap tongues (25, 26) on opposite edges of the panel structure are aligned with respective opposite major faces of the panel structure, and the said bullet-proof layer extends into each said half lap tongue (25, 26), whereby the bullet-proof layers of adjacent like panel structures of the demountable partition system overlap one another. the panel structure comprises a base portion and a panel portion carried on the base portion, the one or more rolling elements (27, 28) are movable between said extended, ground-contacting position and said retracted position within the panel structure using a gearbox mechanism (29, 30), said one or more rolling elements (27, 28) and said gearbox mechanism (29, 30) are self-contained within the base portion, whereby the one or more rolling elements (27, 28) and the gearbox mechanism (29, 30) do not intrude into the panel portion to thereby maintain the integrity of and preserve an operating characteristic of the panel portion, there is provided a reinforced substantially bullet-proof layer between the said two opposite major faces (14, 15) of the panel structure, the said reinforced substantially bullet-proof layer lies in a plane inclined to the family of planes defined by the said two opposite major faces (14, 15) of the panel structure, the panel structure being defined along at least two opposite sides by edges having half lap tongues (25, 26) which overlap one another when two like panel structures are placed in juxtaposition with respect to one another, the said half lap tongues (25, 26) on opposite edges of the panel structure are aligned with respective opposite major faces of the panel structure, and the said bullet-proof layer extends into each said half lap tongue (25, 26), whereby the bullet-proof layers of adjacent like panel structures of the demountable partition system overlap one another.