An electrosurgical system comprising: an electrosurgical forceps including first and second jaw members (110;120) pivotally attached in opposing relation relative to one another, the jaw members relatively movable from a first, open position wherein the jaw members are disposed in spaced relation relative to one another to a second, clamping position wherein the jaw members cooperate to grasp tissue therebetween with a predetermined clamping force, each of the jaw members including an electrically conductive sealing surface; and an electrosurgical generator configured to operatively couple to the electrosurgical forceps and to supply electrosurgical energy to the electrically conductive sealing surfaces, the electrosurgical generator including: at least one input control configured to record a user-defined setting associated with the electrosurgical energy; at least one sensor configured to measure at least one tissue parameter or at least one electrosurgical energy parameter; and a controller configured to predict a tissue mass of the tissue grasped by the jaw members based on the at least one tissue parameter or at least one electrosurgical energy parameter wherein the controller is configured to predict the tissue mass based on the at least one tissue parameter or the at least one electrosurgical energy parameter wherein the tissue mass is predicted prior to application of therapeutic energy to the tissue, and the controller is configured to adjust the user-defined setting as a function of the predicted tissue mass characterised in that the controller is further configured to predict the tissue mass based on total energy delivered to tissue to achieve tissue reaction prior to application of therapeutic energy to the tissue. an electrosurgical forceps including first and second jaw members (110;120) pivotally attached in opposing relation relative to one another, the jaw members relatively movable from a first, open position wherein the jaw members are disposed in spaced relation relative to one another to a second, clamping position wherein the jaw members cooperate to grasp tissue therebetween with a predetermined clamping force, each of the jaw members including an electrically conductive sealing surface; and an electrosurgical generator configured to operatively couple to the electrosurgical forceps and to supply electrosurgical energy to the electrically conductive sealing surfaces, the electrosurgical generator including: at least one input control configured to record a user-defined setting associated with the electrosurgical energy; at least one sensor configured to measure at least one tissue parameter or at least one electrosurgical energy parameter; and a controller configured to predict a tissue mass of the tissue grasped by the jaw members based on the at least one tissue parameter or at least one electrosurgical energy parameter wherein the controller is configured to predict the tissue mass based on the at least one tissue parameter or the at least one electrosurgical energy parameter wherein the tissue mass is predicted prior to application of therapeutic energy to the tissue, and the controller is configured to adjust the user-defined setting as a function of the predicted tissue mass characterised in that the controller is further configured to predict the tissue mass based on total energy delivered to tissue to achieve tissue reaction prior to application of therapeutic energy to the tissue. at least one input control configured to record a user-defined setting associated with the electrosurgical energy; at least one sensor configured to measure at least one tissue parameter or at least one electrosurgical energy parameter; and a controller configured to predict a tissue mass of the tissue grasped by the jaw members based on the at least one tissue parameter or at least one electrosurgical energy parameter wherein the controller is configured to predict the tissue mass based on the at least one tissue parameter or the at least one electrosurgical energy parameter wherein the tissue mass is predicted prior to application of therapeutic energy to the tissue, and the controller is configured to adjust the user-defined setting as a function of the predicted tissue mass characterised in that the controller is further configured to predict the tissue mass based on total energy delivered to tissue to achieve tissue reaction prior to application of therapeutic energy to the tissue.