The assessment of occupational performance in the context of mental and psychosomatic disorders has gained significant importance in various insurance and legal contexts over the past ten years. However, the evaluation of mental and psychosomatic illnesses presents substantial content and methodological challenges, particularly affecting the validity of the expert assessment. The authors of this book have developed and evaluated standards for the assessment of occupational performance and causality issues related to mental and psychosomatic disorders within an interdisciplinary and multicenter collaboration, aiming to make future assessments more methodologically consistent, valid, and transparent in the decision-making process. These standards are recognized by the relevant professional societies.