The turbulent wake behind a truck is responsible for a considerable proportion of the total aerodynamic drag. There is evidence to suggest that the underbody flow affects the wake topology, although this interaction is not well understood. Typical truck trailer underbodies are geometrically very complex and have a range of bluff bodies - such as the wheel and axle assembly, structural beams or the secondary fuel tank for refrigerated trucks - attached. These components block the underbody flow and erode its momentum. However, most of the previous studies of the wake flow have used models with clean underbodies. It is thus uncertain whether the wake shapes found by these studies accurately represent the wake topology behind a real truck with a detailed underbody. The aim of this study is therefore to investigate whether aerodynamic studies working with simplistic underbodies may observe a different wake pattern to those studies that correctly replicate the aerodynamic effects of a realistic underbody. This work uses two models. The first is a simple 1/10th scale model with the basic geometry and aspect ratio of a generic HGV. An underbody roughness pattern, which simulates the effect of the geometric complexity found in typical underbodies and that represents the standard blockage caused by common underbody components, is attached to the underside of the model. The structure of the wake behind the model is studied when the roughness pattern covers an increasing proportion of the total underbody. The observations made are compared to those seen for a configuration with a smooth underbody. The second model consists on a full 1/10th scale truck model with a detailed underbody and rotating wheels. Testing is conducted in a water tow tank, which establishes correct ground conditions. The facility operates at a Reynolds Number of approximately Re = 6.9 x 105. Optical access into the underbody and the near wake is possible through the clear working section of the facility. Stereoscopic Particle Image Velocimetry is used to analyse the flow field. It is found that underbody blockage and / or roughness considerably affects the wake topology and leads to a greater interaction between the low momentum flow in the wake and the higher momentum flow around it. Underbody roughness also increases the size of the region where the low momentum flow is concentrated, which expands laterally outwards away from the model center plane to a greater extent than for the smooth configuration. It is expected that the impact of underbody components on the wake flow structure may also affect the aerodynamic drag associated to the wake. This emphasizes the importance of improving the understanding of the interaction between the underbody and wake flows with drag reduction objectives in mind.