The design value of the toroidal magnetic field (TF) for JET is 3.4 T. Operations were limited to this value until late 1997, when it was raised to 3.8 T. It is planned to further increase it to 4 T. This increase follows an in-depth reassessment of the critical components, namely the TF coils, the Mechanical Structure (MS) and the vacuum vessel (VV).