A system for ablating tissue, said system comprising: an elongate flexible shaft having a longitudinal axis, a proximal portion, a distal portion (112;1112;1212), and a rotation axis; and an ultrasound transducer (114;1120;1220;1320;1420;2116) adjacent the distal end of the elongate flexible shaft, wherein the ultrasound transducer is configured to emit a beam (158;258;1158;1258;1358;1458;2158) of ultrasound energy; and characterised by: a processor, wherein the ultrasound transducer is adapted to operate in an ablation mode and a diagnostic mode, wherein in the ablation mode, the beam of ultrasound energy is configured to ablate target tissue without contact between the ultrasound transducer and the target tissue, wherein, in the ablation mode, the beam of ultrasound energy forms a continuous lesion (260;264;266;268;278;279;308;1260;1360;2172) in the target tissue while the distal portion of the elongate flexible shaft is moving, wherein, in the diagnostic mode, the ultrasound transducer acts as a sensor to measure a reflected ultrasound signal, the reflected ultrasound signal comprising ultrasound energy from the beam of ultrasound energy that is reflected back towards the ultrasound transducer by the target tissue, thereby allowing the ultrasound transducer to sense information about the target tissue, wherein the processor is configured to switch the ultrasound transducer between the ablation mode and the diagnostic mode on a timeshared basis during an ablation procedure in order to allow the ultrasound transducer to sense the information about the target tissue while the ablation procedure is occurring, thereby forming the continuous lesion in the target tissue, and wherein the distal portion of the elongate flexible shaft is movable relative to the longitudinal axis. an elongate flexible shaft having a longitudinal axis, a proximal portion, a distal portion (112;1112;1212), and a rotation axis; and an ultrasound transducer (114;1120;1220;1320;1420;2116) adjacent the distal end of the elongate flexible shaft, wherein the ultrasound transducer is configured to emit a beam (158;258;1158;1258;1358;1458;2158) of ultrasound energy; and characterised by: a processor, wherein the ultrasound transducer is adapted to operate in an ablation mode and a diagnostic mode, wherein in the ablation mode, the beam of ultrasound energy is configured to ablate target tissue without contact between the ultrasound transducer and the target tissue, wherein, in the ablation mode, the beam of ultrasound energy forms a continuous lesion (260;264;266;268;278;279;308;1260;1360;2172) in the target tissue while the distal portion of the elongate flexible shaft is moving, wherein, in the diagnostic mode, the ultrasound transducer acts as a sensor to measure a reflected ultrasound signal, the reflected ultrasound signal comprising ultrasound energy from the beam of ultrasound energy that is reflected back towards the ultrasound transducer by the target tissue, thereby allowing the ultrasound transducer to sense information about the target tissue, wherein the processor is configured to switch the ultrasound transducer between the ablation mode and the diagnostic mode on a timeshared basis during an ablation procedure in order to allow the ultrasound transducer to sense the information about the target tissue while the ablation procedure is occurring, thereby forming the continuous lesion in the target tissue, and wherein the distal portion of the elongate flexible shaft is movable relative to the longitudinal axis. a processor, wherein the ultrasound transducer is adapted to operate in an ablation mode and a diagnostic mode, wherein in the ablation mode, the beam of ultrasound energy is configured to ablate target tissue without contact between the ultrasound transducer and the target tissue, wherein, in the ablation mode, the beam of ultrasound energy forms a continuous lesion (260;264;266;268;278;279;308;1260;1360;2172) in the target tissue while the distal portion of the elongate flexible shaft is moving, wherein, in the diagnostic mode, the ultrasound transducer acts as a sensor to measure a reflected ultrasound signal, the reflected ultrasound signal comprising ultrasound energy from the beam of ultrasound energy that is reflected back towards the ultrasound transducer by the target tissue, thereby allowing the ultrasound transducer to sense information about the target tissue, wherein the processor is configured to switch the ultrasound transducer between the ablation mode and the diagnostic mode on a timeshared basis during an ablation procedure in order to allow the ultrasound transducer to sense the information about the target tissue while the ablation procedure is occurring, thereby forming the continuous lesion in the target tissue, and wherein the distal portion of the elongate flexible shaft is movable relative to the longitudinal axis.