Conveyer belt braking apparatus comprises an idler roller (10) rotatable about an axle, and a brake member (40) mounted adjacent the idler roller and movable relative to the roller between a first, inoperative position adjacent a belt supported by the roller and a second, braking position against the belt. An operating mechanism (26, 28) is arranged to sense the direction of rotation of the roller and to move the brake member from the first, inoperative position to the second, braking position when the direction of rotation of the roller reverses in use. Preferably the mechanism includes a clutch bearing (26) that engages to move the brake member from the first inoperative position to the second, braking position thereof when the roller reverses direction.