A detailed experimental study of two-dimensional and three-dimensional separated flow behind a normal fence, mounted at the front of a long splitter plate, is reported. The Reynolds number, based on the height of the fence above the splitter plate, was 3800. Results from numerical simulations of the two-dimensional flow, involving eddy viscosity 'k-epsilon' and Reynolds stress turbulence models, are also presented. A systematic approach has been used to extend earlier work on two-dimensional coplanar separated flows to three-dimensional flows using a downstream facing v-shaped separation line. The 'arms' of this swept geometry are wide enough to provide two spanwise-invariant regions where the flow is two-dimensional, but is not coplanar because of the existence of lateral velocity. In these regions, six of the mean rates of strain are non-zero compared to the four that are non-zero in the unswept flow. In the central part of the flow, where the two lateral inflows meet and decelerate, the flow is three-dimensional and all nine mean rates of strain are non-zero. Extensive use of pulsed-wire anemometry techniques to determine mean and fluctuating quantities in an unswept separated flow and in the spanwise invariant region of the swept flow revealed that, for the sweep angle considered (10 degrees), the two separation bubbles are very similar in most respects. For the swept case, the bulk of the flow is convected sideways (parallel to the separation line) at a roughly uniform velocity, with the lateral velocity going to zero rapidly near the surface. The component of vorticity perpendicular to the separation line at separation and the additional strain rates such as dW / dy associated with the swept flow appear to have only a small effect on turbulence levels. In the central, fully three-dimensional part of the flow, significant increases in the bubble size were observed, owing to the swelling effect of the two lateral inflows. The increased size of the bubble was accompanied by dramatic increases in Reynolds normal and shear stresses. Examination of the extra rates of strain in the central region indicated that the strain ratio (dW / dz) I(dU / dy) was having a critical role in governing the turbulence levels.