It is well known that a wake will develop downstream of a tidal stream turbine due to extraction of axial momentum across the rotor plane. To select a suitable layout for an array of horizontal axis tidal stream turbines it is important to understand the extent and structure of the wakes of each turbine. Studies of wind turbines and isolated tidal stream turbines have shown that the velocity reduction in the wake of a single device is a function of the rotor operating state (specifically thrust) and that the rate of recovery of wake velocity is dependent on mixing between the wake and the surrounding flow. For an unbounded flow the velocity of the surrounding flow is similar to that of the incident flow. However, the velocity of the surrounding flow will be increased by the presence of bounding surfaces formed by the bed and the free surface and by adjacent devices. This paper presents the results of a study investigating the influence of such bounding surfaces on the structure of the wake of tidal stream turbines.