An apparatus for effecting surgery by hyperthermia comprising: a heat source (20, 21) arranged to apply heat to a tumour volume within a patient on whom the surgery is to be effected, the tumour volume having a peripheral surface; a non-invasive detection system arranged to generate a series of output signals over a period of time representative of temperature in the patient at the peripheral surface of the tumour volume as the temperature of the tumour volume changes during that time; a control system comprising: a first means arranged to identify a plurality of locations in the tumour volume to be heated to a required hyperthermic temperature; a second means arranged to use the output signals to monitor the temperature at a plurality of locations in the tumour volume as the temperature changes over the period of time; and a third means arranged to control the heat source to effect heating of a plurality of selected areas of the tumour volume; the control system being arranged in response to said temperatures at the surface of the tumour volume to operate the third means to control the selection of the area of the tumour volume to which heat is applied and to control the amount of heat applied to the area of the tumour volume such that heating is continued until the temperature of the surface of the tumour volume at a certain one of the selected areas is raised to a required hyperthermic temperature. a heat source (20, 21) arranged to apply heat to a tumour volume within a patient on whom the surgery is to be effected, the tumour volume having a peripheral surface; a non-invasive detection system arranged to generate a series of output signals over a period of time representative of temperature in the patient at the peripheral surface of the tumour volume as the temperature of the tumour volume changes during that time; a control system comprising: a first means arranged to identify a plurality of locations in the tumour volume to be heated to a required hyperthermic temperature; a second means arranged to use the output signals to monitor the temperature at a plurality of locations in the tumour volume as the temperature changes over the period of time; and a third means arranged to control the heat source to effect heating of a plurality of selected areas of the tumour volume; the control system being arranged in response to said temperatures at the surface of the tumour volume to operate the third means to control the selection of the area of the tumour volume to which heat is applied and to control the amount of heat applied to the area of the tumour volume such that heating is continued until the temperature of the surface of the tumour volume at a certain one of the selected areas is raised to a required hyperthermic temperature. a first means arranged to identify a plurality of locations in the tumour volume to be heated to a required hyperthermic temperature; a second means arranged to use the output signals to monitor the temperature at a plurality of locations in the tumour volume as the temperature changes over the period of time; and a third means arranged to control the heat source to effect heating of a plurality of selected areas of the tumour volume; the control system being arranged in response to said temperatures at the surface of the tumour volume to operate the third means to control the selection of the area of the tumour volume to which heat is applied and to control the amount of heat applied to the area of the tumour volume such that heating is continued until the temperature of the surface of the tumour volume at a certain one of the selected areas is raised to a required hyperthermic temperature.