Around 50% of steel and steel—concrete composite bridges owned by Trafikverket have been built before 1970, when there was limited knowledge about fatigue and brittle fracture of structural steel products. This was the case in particular for the usage of these products in construction of welded bridges. At the same time, fatigue deterioration in these structures is undergoing due to the escalation of freight volume as well as the increased axle loads of modern trains and lorries. Realistic assessment of residual service life of these structures is crucial in more efficient planning of repairs and avoiding unnecessary replacement costs. Therefore, more accurate condition assessment methods are important from both economic and sustainability aspects. While the improved assessment methods for existing structures have been subject of extensive research since 1990's, their use in regular engineering practice is still limited. This study is an attempt to review the recent advances in structural assessment of welded steel bridges and to present them in a structured manner for the use of practicing engineer. In this report, a multi-step assessment methodology for fatigue and brittle fracture of welded bridges is presented, mainly based on the past research in Europe. The assessment steps are comprised of preliminary assessment (phase I), detailed investigation (phase II), and expert investigation (part III). Detailed information is given in the report for the corresponding input data (resistance-side and action-side) and modelling options for each of these assessment steps. As expected, the more advanced method is used, the more extensive set of input data and analytical effort is required. When applicable, the assessment method has been adapted to the relevant technical requirements stipulated by Trafikverket. Finally, the application of the presented multi-step assessment is examined on a case study bridge (Göta river bridge). Phase II assessment of the bridge based on historical loading data shows insufficient fatigue life for a critical detail that has been replicated in many locations over the bridge. The results of phase III assessment suggest that intensified inspections of vulnerable details in regular time intervals are sufficient for ensuring the safe service of the structure until the end of its planned service life.