An electrosurgical instrument for delivering radiofrequency electromagnetic energy and microwave EM energy for resection and ablation of biological tissue, the instrument comprising: an energy conveying cable structure comprising: a coaxial transmission line for conveying microwave energy, the coaxial transmission line for conveying the microwave energy is a first coaxial transmission line, the coaxial transmission line comprising an inner conductive layer, an outer conductive layer and a dielectric layer separating the inner conductive layer from the outer conductive layer, wherein the inner conductive layer is formed around a hollow longitudinal passage along the cable structure; and a transmission line for conveying radiofrequency energy along the hollow longitudinal passage, the transmission line for conveying the radiofrequency energy is a second coaxial transmission line, wherein the second coaxial transmission line comprises an innermost conductive element extending through the longitudinal passage, the inner conductive layer, and an innermost dielectric layer separating the inner conductive layer from the innermost conductive element; and an instrument tip at a distal end of the energy conveying cable structure, the instrument tip comprising: a dielectric tip element extending longitudinally beyond a distal end of the outer conductive layer, wherein the inner conductive layer extends longitudinally beyond the distal end of the outer conductive layer within the dielectric tip element to form a microwave radiator; and an active electrode and a return electrode mounted at a distal end of the dielectric tip, wherein the active electrode and the return electrode are connected to the transmission line to support an RF field therebetween at the distal end of the dielectric tip element, characterized in that the active electrode is electrically connected to the inner conductive layer and the return electrode is electrically connected to the innermost conductive element, wherein the innermost conductive element and the outer conductive layer are electrically grounded. an energy conveying cable structure comprising: a coaxial transmission line for conveying microwave energy, the coaxial transmission line for conveying the microwave energy is a first coaxial transmission line, the coaxial transmission line comprising an inner conductive layer, an outer conductive layer and a dielectric layer separating the inner conductive layer from the outer conductive layer, wherein the inner conductive layer is formed around a hollow longitudinal passage along the cable structure; and a transmission line for conveying radiofrequency energy along the hollow longitudinal passage, the transmission line for conveying the radiofrequency energy is a second coaxial transmission line, wherein the second coaxial transmission line comprises an innermost conductive element extending through the longitudinal passage, the inner conductive layer, and an innermost dielectric layer separating the inner conductive layer from the innermost conductive element; and a coaxial transmission line for conveying microwave energy, the coaxial transmission line for conveying the microwave energy is a first coaxial transmission line, the coaxial transmission line comprising an inner conductive layer, an outer conductive layer and a dielectric layer separating the inner conductive layer from the outer conductive layer, wherein the inner conductive layer is formed around a hollow longitudinal passage along the cable structure; and a transmission line for conveying radiofrequency energy along the hollow longitudinal passage, the transmission line for conveying the radiofrequency energy is a second coaxial transmission line, wherein the second coaxial transmission line comprises an innermost conductive element extending through the longitudinal passage, the inner conductive layer, and an innermost dielectric layer separating the inner conductive layer from the innermost conductive element; and an instrument tip at a distal end of the energy conveying cable structure, the instrument tip comprising: a dielectric tip element extending longitudinally beyond a distal end of the outer conductive layer, wherein the inner conductive layer extends longitudinally beyond the distal end of the outer conductive layer within the dielectric tip element to form a microwave radiator; and an active electrode and a return electrode mounted at a distal end of the dielectric tip, wherein the active electrode and the return electrode are connected to the transmission line to support an RF field therebetween at the distal end of the dielectric tip element, characterized in that the active electrode is electrically connected to the inner conductive layer and the return electrode is electrically connected to the innermost conductive element, wherein the innermost conductive element and the outer conductive layer are electrically grounded. an active electrode and a return electrode mounted at a distal end of the dielectric tip, wherein the active electrode and the return electrode are connected to the transmission line to support an RF field therebetween at the distal end of the dielectric tip element, characterized in that the active electrode is electrically connected to the inner conductive layer and the return electrode is electrically connected to the innermost conductive element, wherein the innermost conductive element and the outer conductive layer are electrically grounded.