The underbody of a truck is responsible for an appreciable portion of the vehicle’s aerodynamic drag, and thus its fuel consumption. This paper investigates experimentally the flow around side-skirts, a common underbody aerodynamic device which is known to be effective at reducing vehicle drag. A full, 1/10 scale European truck model is used. The chassis of the model is designed to represent one that would be found on a typical trailer, and is fully reconfigurable. Testing is carried out in a water towing tank, which allows the correct establishment of the ground flow and rotating wheels. Optical access into the underbody is possible through the clear working section of the facility. Stereoscopic and planar Particle Image Velocimetry (PIV) set-ups are used to provide both qualitative images of and quantitative information on the flow field. The planar PIV set-up employs novel techniques developed in-house, including the use of two laser sheets to reduce shadow regions, and frame averaging such that presented results are produced using an average of up to 1800 independent data points. It is shown that one drag-reducing mechanism of side-skirts at zero-degree yaw is the prevention of high-momentum flow at the side of the vehicle entering the upper half of the underbody. However, the flow topology varies significantly with vertical position, and side-skirts are shown to have effects on near-ground flow which could have a detrimental effect on vehicle drag. It is therefore likely that an optimum value exists for the height of a side-skirt from the ground.