This invention relates to braking mechanism for a downhole tool such as a steering tool. The braking mechanism comprises:a body adapted for mounting upon the tool;at least one braking member which is movably mounted upon the body;a resilient biasing means biasing the braking member away from the body; anda damping mechanism connected to the braking member, the damping mechanism providing a damping force opposing movements of the braking member relative to the body, the damping force being dependent upon the rate of movement of the braking member.