Accurate position data show that Brunt Ice Shelf has an irregular three-dimensional motion driven by the ocean tide. A 30-day GPS record from Halley station shows not only the expected vertical tidal displacement, but also a significant variation in horizontal velocity. The maximum forward velocity occurs about four hours before the maximum tidal height, a phase relationship that suggests ocean currents may be responsible. However, values of ice/ water friction coefficients needed to make this explanation plausible appear to be too high to be physically reasonable. Alternative explanations invoke interactions between the ice shelf and a grounded area at the ice front. Our findings suggest that significant errors might result if these motions are ignored when interpreting ‘double difference’ satellite SAR interferograms, where the common assumption is that the horizontal velocity of an ice shelf has no tidal dependence.