Abstract: A tool can be released from a tool string in a wellbore by setting the tool and applying reverse torque to the tool string for an extended period. The release mechanism (200) comprises a latch (250) including a plurality of collects (253) which are held in a radially extended position to secure a tool "T" to the tool string. A first member (201) is connected to the tool string and to the latch (250). A second member (202) is connected to the latch. In use, after the tool is set reverse torque is applied to the first member (201) to rotate it relative to the second member (202) which is constrained against rotation by the action of the tool on the latch. When the first member (201) has rotated sufficiently the first and second members occupy a release position in which downward movement of the first member (201) relative to the second member (202) (which was previously blocked) is permitted. This movement releases the latch (250) and hence allows separation of the tool and the tool string.