The book "Biaxial Testing for Fabrics and Foils" provides a comprehensive and critical analysis of current design practices for tensioned membrane structures. It addresses the experimental data necessary for the development of design codes and testing procedures, highlighting the challenges associated with the lack of standardized methods. The technical requirements for biaxial testing devices are examined in detail, and key aspects that must be considered in the development of testing procedures are emphasized. The analysis is supported by results from a comparison of biaxial testing machines involving leading research laboratories. Additionally, the design and implementation of a biaxial testing device for architectural fabrics at the Politecnico di Milano is presented, which is considered a benchmark in this field. The latest developments in biaxial testing are also discussed.