Fuel assemblies (FAs) in light water reactors (LWRs) are engineered for high robustness and reliability, delivering strong performance under normal and demanding operating conditions, such as power uprates, high burnups, extended cycle lengths of up to 24 months, and challenging water chemistry environments. Since the mid-2000s, and especially following the Fukushima accident, significant advancements have been made in FA design, safety analysis, and testing methodologies to better understand structural behaviour and demonstrate mechanical integrity in water-cooled reactors. This publication covers design considerations, safety and design analysis, operational and handling experience, and the in-reactor structural behaviour of LWR fuel assemblies.