The rapid processing of glass mat thermoplastics (GMT) is well known, large complex parts can be processed in less than one minute. It is however important that the material is produced properly to avoid a reduction in mechanical properties. There are some underlying industrial questions for this work. The first, how should the material be put in the mould and how does the flow affect the properties? The second question is if voids are detrimental for the mechanical performance of the material. The two papers of this thesis deal with some common problems during processing of GMT. Effects of variables such as voids, fibre orientation and fibre content variations on properties have been studied. Paper I deals with the flow during the mould-filling step. The work focused on the effect of flow on mechanical properties for two commercial GMT materials. The influence of variations in fibre orientation and fibre content are included. The stiffness has been measured for both specimens moulded with or without flow during compression. The result shows that flow has a significant effect on fibre orientation which in turn has a great effect on the mechanical properties. The fibre content seems to be more or less unaffected by the flow for the tested geometry. Paper II deals with another typical problem during GMT processing which is the presence of voids. To examine the effect voids on properties, materials with different void contents have been moulded. The effect of voids on damage initiation and propagation is investigated and the results show that voids are not a large problem in GMT. Cracks propagate in the same manner in void rich as in void free materials.