A sensor patch for attachment to a patient's body for obtaining physiological data from the patient and transmitting the physiological data to monitoring equipment. The sensor patch includes a structural support for supporting the sensor patch on the patient's body, a sensor for sensing physiological data from the patient's body, an adhesive for attaching the sensor patch to the patient's body, an electronics package including a power supply and circuitry for processing the sensed data to produce a conditioned signal, communications equipment for passing the sensed data from the sensor to the electronics package, and a transmitter for transmitting the conditioned signal from the sensor patch to the monitoring equipment. In a preferred embodiment, the electronics package and transmitter form part or parts of the sensor patch itself, and the sensor comprises one or more thermistors to sense the temperature of the patient's body. In another embodiment, the transmitter is an infra-red transmitter. There is also provided an ECG monitoring system incorporating the sensor patch.