Commission X aims to establish a practical procedure for assessing the strength and integrity of welded structures in design as well as in service with known or assumed flaws, where attention is paid to the role and influence of welding residual stresses, strength mismatch between base and weld metals, and heterogeneity in toughness of welds. Recent activity has focused on the development of a practical guideline for Fitness-for-Service (FFS) assessment for welded components containing a flaw and damage, which includes stress/strain-based assessment, constraint analysis and toughness testing procedure for welds. With further development of FFS for welds, Commission X faces the challenge of advanced design of transport vehicles and infrastructures such as energy plants, pipelines, bridges and buildings constructed with high-performance steels and high welding technologies. The C-X FFS operates as a “supplement” to the existing standards/guidelines like BS7910, R6 and API 579-1/ASME FFS-1, and consists of specific subjects that are not covered by those standards/guidelines. The C-X FFS gives state-of-the-art knowledge, advice and/or recommendations on the structural performance assessment of welds with respect to the avoidance of brittle, ductile and environment induced fracture.