Market trends within the automotive industry are leading to an ever-increasing use of high-strength and advanced high-strength steels (AHSS). The attraction of these materials is the advantage of excellent formability, combined with increasingly high tensile strength. It is a well-known fact that weld performance can be an issue with AHSS, where susceptible weld microstructures can lead to low strengths and undesirable failure modes. Much research has been conducted and published on this subject. A less well-documented effect in the weld performance of AHSS is ‘ageing’, whereby a weld exhibits poor mechanical performance immediately after welding, but after a certain period of time, the weld properties improve significantly. In the Corus — SMI research cooperation, this ‘ageing’ effect was first observed in weld samples in 2004, since this time ageing has been a major topic of combined research. This presentation is a summary of the Corus — SMI weld ageing study, highlighting the major issues and characteristics of the effect: the influence of process parameters, the susceptible alloying systems and the possible mechanisms that can cause ageing of the weld. The ‘ageing’ effect has serious implications for standardized testing procedures, where the timescale between welding and testing is not specified, ageing can have a huge influence on the welding results obtained in ageing susceptible materials. The final aspect of this report is to consider the consequences of ageing for weld testing procedures.