An electronic pointing cursor control device such as a trackball or mouse, comprising a first chamber adjoined to a second chamber, the chambers being separated by a fluid-tight wall comprising an optically permeable region; wherein the first chamber contains electronic components including optical detectors directed towards the optically permeable region and a ball protruding through an aperture provided in the second chamber, the detectors being operable in use to detect and generate electronic signals representative of motion of the ball. Also provided is an electronic pointing or cursor control device comprising a ball and optical detectors operable to detect and generate electronic signals representative of motion of the ball, wherein the detectors are mounted such that a radial line from the ball to the detectors forms a non-zero angle with a diametric line though the ball normal to the mounting plane of the device.