A method for non-invasive temperature determination in biological tissue treated with treatment radiation wherein a reaction which depends on the treatment temperature is generated and detected. Measurement radiation pulses and pulses of the treatment radiation are applied to the tissue to be treated successively, in particular in an alternating way, from the same radiation source. A device includes a radiation source, an irradiation lens system, detectors for determining a reaction and a system control suitable for controlling treatment radiation and measurement radiation pulses. The system control generates successive, in particular alternating, pulses of treatment radiation and measurement radiation pulses from one radiation source only, in particular a laser which can be modulated.
