A microwave ablation system having: an elongate microwave ablation probe (101, 301, 401, 501, 601, 701, 901) comprising: a probe body(111, 311, 411, 511, 611, 711) comprising a radiation window (115, 315, 415, 515, 615, 715) that is at least partially transparent to microwave energy; an antenna (103, 303, 403, 503, 903) located within the probe body, the antenna having a radiating portion (105, 305, 405, 505, 605, 705, 905) for emission of microwave energy at a distal portion (114, 314, 414, 514, 614, 714) of the probe body, wherein the radiating portion (105, 305, 405, 505, 605, 705, 905) is aligned with the radiation window (115, 315, 415, 515, 615, 715); a cable located within the probe body and connected to the antenna (103, 303, 403, 503, 903), the cable comprising at least one conductor; a first electrode located along the probe body; and a second electrode located along the probe body; a microwave energy source connected to the antenna (103, 303, 403, 503, 903) via the cable; a radiofrequency energy source connected to the first electrode with a first lead and connected to the second electrode with a second lead; a reference patch to be located on the patient's skin; and a sensing circuit connected to the first electrode and second electrode, the sensing circuit being configured to generate at least one signal corresponding to an impedance of tissue, the system being configured to use the impedance of tissue to detect a change in tissue due to microwave ablation; wherein the first electrode is a first distal electrode (131, 331, 431, 531, 661, 771) located along the probe body distal to the radiation window (115, 315, 415, 515, 615, 715) and the second electrode is a first proximal electrode (141, 341, 441, 541, 641, 741) located along the probe body proximal to the radiation window; and wherein the sensing circuit is configured to sense a first impedance between the first electrode and the reference patch and to sense a second impedance between the second electrode and the reference patch. an elongate microwave ablation probe (101, 301, 401, 501, 601, 701, 901) comprising: a probe body(111, 311, 411, 511, 611, 711) comprising a radiation window (115, 315, 415, 515, 615, 715) that is at least partially transparent to microwave energy; an antenna (103, 303, 403, 503, 903) located within the probe body, the antenna having a radiating portion (105, 305, 405, 505, 605, 705, 905) for emission of microwave energy at a distal portion (114, 314, 414, 514, 614, 714) of the probe body, wherein the radiating portion (105, 305, 405, 505, 605, 705, 905) is aligned with the radiation window (115, 315, 415, 515, 615, 715); a cable located within the probe body and connected to the antenna (103, 303, 403, 503, 903), the cable comprising at least one conductor; a first electrode located along the probe body; and a second electrode located along the probe body; a probe body(111, 311, 411, 511, 611, 711) comprising a radiation window (115, 315, 415, 515, 615, 715) that is at least partially transparent to microwave energy; an antenna (103, 303, 403, 503, 903) located within the probe body, the antenna having a radiating portion (105, 305, 405, 505, 605, 705, 905) for emission of microwave energy at a distal portion (114, 314, 414, 514, 614, 714) of the probe body, wherein the radiating portion (105, 305, 405, 505, 605, 705, 905) is aligned with the radiation window (115, 315, 415, 515, 615, 715); a cable located within the probe body and connected to the antenna (103, 303, 403, 503, 903), the cable comprising at least one conductor; a first electrode located along the probe body; and a second electrode located along the probe body; a microwave energy source connected to the antenna (103, 303, 403, 503, 903) via the cable; a radiofrequency energy source connected to the first electrode with a first lead and connected to the second electrode with a second lead; a reference patch to be located on the patient's skin; and a sensing circuit connected to the first electrode and second electrode, the sensing circuit being configured to generate at least one signal corresponding to an impedance of tissue, the system being configured to use the impedance of tissue to detect a change in tissue due to microwave ablation; wherein the first electrode is a first distal electrode (131, 331, 431, 531, 661, 771) located along the probe body distal to the radiation window (115, 315, 415, 515, 615, 715) and the second electrode is a first proximal electrode (141, 341, 441, 541, 641, 741) located along the probe body proximal to the radiation window; and wherein the sensing circuit is configured to sense a first impedance between the first electrode and the reference patch and to sense a second impedance between the second electrode and the reference patch.