Numerous technical documents have been published by national and international authorities that focus on the systematic and scientific methods that can be used to accurately assess the residual strength, durability and reliability of structural materials, assemblies and systems in existing buildings. In the scope of timber structures, they need to be revised, expanded and enhanced regarding the new knowledge in the field of material sciences, technological advancement of structural assessment and monitoring and practical experience regarding the professional work on existing structures. Currently, the assessment of existing structures focuses mainly on the identification, localization and evaluation of deterioration and damage. The diagnoses are based on the design procedures of new structures and the planning of interventions often suggests reinforcement. The current practice of the assessment of existing structures might not be considered suitable to facilitate confident decisions about the reliability of structures. Within the ARES (Assessment and Rehabilitation of Existing Structures) project, which is funded by Croatian Science Foundation, the main goal is to study the role of assessment on the reliability analysis of existing structures. The project focuses on existing timber and masonry structures to evaluate the benefits of assessment regarding the economy and safety of structures.