Energy-based tissue sealing or tissue cutting device (101, 601) having a tissue sealing zone (105, 605); the device comprising: an energy delivery unit (108, 109, 608, 609) for delivering energy to a tissue region (104, 604) within the tissue sealing zone (105, 605); an optical source (102, 602) configured to irradiate at least a portion of the tissue sealing zone with an optical probe beam; at least one optical detector (103, 603, 610) configured to receive at least a portion of the optical probe beam that has been returned from within the tissue sealing zone; and an optical analysis unit configured to: generate an optical scattering coefficient indicative of optical scattering in the tissue sealing zone from the received optical probe beam; and characterised in that the optical analysis unit is further configured to indicate an onset of tissue sealing based on the successive occurrence in time of a turning point, a point of inflection, and a second turning point in the optical scattering coefficient, wherein the second turning point succeeds the point of inflection, and to indicate a termination of tissue sealing; wherein the termination of tissue sealing is indicated based on a third turning point in the optical scattering coefficient, wherein the third turning point in the optical scattering coefficient succeeds the second turning point. an energy delivery unit (108, 109, 608, 609) for delivering energy to a tissue region (104, 604) within the tissue sealing zone (105, 605); an optical source (102, 602) configured to irradiate at least a portion of the tissue sealing zone with an optical probe beam; at least one optical detector (103, 603, 610) configured to receive at least a portion of the optical probe beam that has been returned from within the tissue sealing zone; and an optical analysis unit configured to: generate an optical scattering coefficient indicative of optical scattering in the tissue sealing zone from the received optical probe beam; and characterised in that the optical analysis unit is further configured to indicate an onset of tissue sealing based on the successive occurrence in time of a turning point, a point of inflection, and a second turning point in the optical scattering coefficient, wherein the second turning point succeeds the point of inflection, and to indicate a termination of tissue sealing; wherein the termination of tissue sealing is indicated based on a third turning point in the optical scattering coefficient, wherein the third turning point in the optical scattering coefficient succeeds the second turning point. generate an optical scattering coefficient indicative of optical scattering in the tissue sealing zone from the received optical probe beam; and characterised in that the optical analysis unit is further configured to indicate an onset of tissue sealing based on the successive occurrence in time of a turning point, a point of inflection, and a second turning point in the optical scattering coefficient, wherein the second turning point succeeds the point of inflection, and to indicate a termination of tissue sealing; wherein the termination of tissue sealing is indicated based on a third turning point in the optical scattering coefficient, wherein the third turning point in the optical scattering coefficient succeeds the second turning point.