The paper describes some of the work undertaken within the Demopipe X100 project for the evaluation of X100 pipeline steels. The specific areas of interest described are the development of suitable girth welding procedures using mechanised GMAW (gas metal-arc welding) and SMAW (shielded metal arc welding) processes, the resultant joint properties and the assessment of defect tolerance levels applicable to X100 line pipe and girth welds. Numerous weld metal chemistries were examined with the two welding processes typical of field-welding construction, such that various strength, toughness and hardness levels could be assessed. Preheating and interpass temperature requirements for the various consumable and base material combinations were assessed. The final output of the consumable evaluation programme resulted in the selection of several weld metal chemical compositions and welding procedures suitable for the in-field construction of two X100 pipe strings for burst testing. The resultant field weldability and joint integrity was proved through their exemplary performance in the two burst tests conducted within the project. The defect tolerance work centred on the production of wide plate tests and full scale bend tests in conjunction with numerous small scale CTOD (crack tip opening displacement) and impact toughness tests, all of which incorporated mechanised GMAW and SMAW girth welds. Assessment of the data using classical failure assessment diagrams has led to a conservative result as is commonly found for pipeline applications. The results of the curved wide plate testing undertaken have, in general, indicated that it will be possible to obtain adequate levels of defect tolerance in X100 pipeline girth welds, although in the short term project specific testing may be required to optimise the acceptance criteria for a specific application and design conditions.