Pressure welds occur in extruded aluminium profiles either as longitudinal welds (seam welds) or as transverse welds (charge welds). While longitudinal welds only occur in hollow profiles, transverse welds always occur when the billet is extruded on top of a metal rest left in the die from the previous extrusion, i.e. in extrusion of hollows as well as in billet-to-billet extrusion, and in pre-chamber extrusion of solid profiles. Longitudinal welds are formed as the metal streams of different die ports gather behind each die-bridge and join. They extend longitudinally through the profile in the location behind each die web. Transverse welds form as the front end face of each billet pressure welds to the sheared-off back-end face of the extrusion residue left inside the die from the previous extrusion. Transverse welds commonly have a tongue-like shape inside the resulting extrusion and often extend over a large length of the extrusion. Since extrusion welds are formed by solid pressure-welding, they are produced with mechanical properties corresponding to the rest of the profile. Unlike fusion welding, there are no filler metal or soft heat-affected zones. But in spite of this, problems in terms of poor weld quality sometimes occur in industrial production of aluminium profiles. Extrusion welds of insufficient quality contain no voids or cavities and could, therefore, be difficult to detect by non-destructive tests as ultrasonic or x-ray inspection. Since extrusion welds defects are often unfavourable conditions in the extrusion process, they can to a large extent be eliminated through corrective measures taken by the extruder. This paper reviews the existing knowledge, as regards formation of extrusion welds, and defects in such welds. Preventive measures that can be made by the extruder to avoid formation of such defects are also discussed.