Abstract The present paper reviews the concept of flexibility for an application to power systems for operational planning of transmission grid in market environment. The background that makes useful the application of the flexibility notion to power systems is explained. Some general definitions concerning the concept of flexibility in more recent scientific literature are reported. A specific and unique definition of transmission system flexibility in respect of the generation changes is proposed. The mathematical model and the calculation methodology of some bulk system flexibility indices are presented. A dedicated tool in Matlab for their computation is briefly shown in its features. The tool is owned by the Italian Transmission System Operator (TERNA) and is in testing for the flexibility evaluation of parts of the Italian National Transmission Grid. An application of the procedure to a real case is presented and the main results are discussed with the aim of test the validity of the proposed indices.