An electrosurgical apparatus, comprising: an electrosurgical element; and a control system configured to: electrically drive the electrosurgical element in a first electrical driving mode, the first electrical driving mode being such as to cause heating of human or animal tissue by the electrosurgical element when the apparatus is used in a surgical operation, the heating contributing to modification or cutting of tissue by the electrosurgical element; electrically drive the electrosurgical element in a second electrical driving mode; and measure an electrical response of the electrosurgical element during the electrical driving of the electrosurgical element in the second electrical driving mode, wherein the control system is further configured to use the measured electrical response to determine compositional information about material that is or was in thermal contact with the electrosurgical element during the driving of the electrosurgical element in the second electrical driving mode, characterized in that: the electrical driving of the electrosurgical element in the second driving mode is such as to generate heating predominantly by resistive heating within a resistive element of the electrosurgical element and the measuring of the electrical response of the electrosurgical element during the electrical driving of the electrosurgical element in the second electrical driving mode comprises measuring an electrical response of the resistive element to the heating generated within the resistive element. an electrosurgical element; and a control system configured to: electrically drive the electrosurgical element in a first electrical driving mode, the first electrical driving mode being such as to cause heating of human or animal tissue by the electrosurgical element when the apparatus is used in a surgical operation, the heating contributing to modification or cutting of tissue by the electrosurgical element; electrically drive the electrosurgical element in a second electrical driving mode; and measure an electrical response of the electrosurgical element during the electrical driving of the electrosurgical element in the second electrical driving mode, wherein the control system is further configured to use the measured electrical response to determine compositional information about material that is or was in thermal contact with the electrosurgical element during the driving of the electrosurgical element in the second electrical driving mode, characterized in that: the electrical driving of the electrosurgical element in the second driving mode is such as to generate heating predominantly by resistive heating within a resistive element of the electrosurgical element and the measuring of the electrical response of the electrosurgical element during the electrical driving of the electrosurgical element in the second electrical driving mode comprises measuring an electrical response of the resistive element to the heating generated within the resistive element. electrically drive the electrosurgical element in a first electrical driving mode, the first electrical driving mode being such as to cause heating of human or animal tissue by the electrosurgical element when the apparatus is used in a surgical operation, the heating contributing to modification or cutting of tissue by the electrosurgical element; electrically drive the electrosurgical element in a second electrical driving mode; and measure an electrical response of the electrosurgical element during the electrical driving of the electrosurgical element in the second electrical driving mode, wherein the control system is further configured to use the measured electrical response to determine compositional information about material that is or was in thermal contact with the electrosurgical element during the driving of the electrosurgical element in the second electrical driving mode, characterized in that: the electrical driving of the electrosurgical element in the second driving mode is such as to generate heating predominantly by resistive heating within a resistive element of the electrosurgical element and the measuring of the electrical response of the electrosurgical element during the electrical driving of the electrosurgical element in the second electrical driving mode comprises measuring an electrical response of the resistive element to the heating generated within the resistive element.