Friction Stir Welding is a solid-state welding process invented by TWI in 1991. The FSW process is unique in the sense that joining of un-weldable alloys readily can be made. The thermomechanical conditions present in the workpiece during the welding process are of great interest since these control the properties of the weld. In the present work, a set of experimental, analytical and numerical analyses are carried out in order to evaluate the thermomechanical conditions descriptive for welding of aluminium, in this case AA2024-T3, under a specific set of welding parameters. Despite these specific data, the developed models can be applied for other alloys and welding parameters as well. A detailed experiment is carried out which constitutes the basis for the development and validation of the numerical and analytical models presented in this work. The contact condition at the tool/matrix interface is an important parameter for the different models presented. In the welds, a shear layer is observed along the tool/matrix interface. Based on the experiment, it is suggested that for these experimental welds, the contact condition is most likely sticking. A new analytical model for estimation of the heat generation in FSW is presented. This model demonstrates the flexibility regarding different contact conditions. Furthermore, the analytical model can estimate the heat generation from tools having conical surfaces such as a conical shoulder or a threaded probe. A numerical procedure for applying this analytical heat source model along the tool/matrix interface is proposed using the FE software FEMLAB. Both steady-state and transient models are presented. The fully coupled thermomechanical model developed in ABAQUS/Explicit is the most advanced of the presented models in the sense that the heat generation and the material flow is part of the solution. Secondly, the model is characterized by having a minimum of described boundary conditions. The material flow around the probe is examined using a 2D flow model developed in FEMLAB. Of special interest is the shear layer at the probe/matrix interface. Different aspects of the flow behavior in this region are characterized. The flow fields in a set of experimental welds are visualized by inserting marker material within the workpieces. By welding through the marker material a dispersion/deformation of this material takes place. This indicates the flow pattern in the region surrounding the probe. The marker material is then illuminated during a subsequent X-ray tomography examination. 2D and 3D computational tomography models reveal information regarding the material flow which can not be obtained through traditional optical metallography. However, the latter is used for illustration of microstructural changes which also provides additional information about the material flow.