Abstract: A tamper resistant fastener includes a body (10) of substantially circular cross-section having a plurality of depressions (15?) formed in its surface. Each depression (15?) has a curved concave surface (16?) at one end and a curved convex surface (17?) at its other end. The depressions are engageable by a tool whereby the fastener can be turned in one direction by the tool to engage the fastener and, once engaged, the fastener can only be turned in the opposite, unfastening direction when engaged by the tool. The fastener may take the form of a nut or a screw or a screw-threaded bolt.