fib Bulletin 40 deals mainly with the use of FRP bars as internal reinforcement for concrete structures. The background of the main physical and mechanical properties of FRP reinforcing bars is presented, with special emphasis on durability aspects. For each of the typical ultimate and serviceability limit states, the basic mechanical model is given, followed by different design models according to existing codes or design guidelines. Composite FRP materials are still relatively new in construction and most engineers are unfamiliar with their properties and characteristics. The second chapter of this bulletin therefore aims to provide practising engineers with the necessary background knowledge in this field, and also presents typical products currently available in the international market. The third chapter deals with the issue of durability and identifies the parameters that can lead to deterioration, which is necessary information when addressing design issues. A series of parameters is used to identify the allowable stress in the FRP after exposure for a specified period of time in a specific environment. The bulletin covers the issues of Ultimate Limit States (primarily dealing with flexural design), Serviceability Limit States (dealing with deflections and cracking), Shear and Punching Shear and Bond and Tension Stiffening. It provides not only the state-of-the-art but also in many cases ideas for the next generation of design guidelines. The final chapter deals with the fundamental issue of design philosophy. The use of these new materials as concrete reinforcement has forced researchers to re-think many of the fundamental principles used until now in RC design. The bulletin ends with a discussion of a possible new framework for developing partial safety factors to ensure specific safety levels that will be flexible enough to cope with new materials.