Tissue classifying apparatus in which forward microwave radiation (e.g. having a frequency 500 MHz to 60 GHz) is supplied from a source (108) along a first transmission path to a probe (116) which delivers it into tissue to be classified. The probe (116) receives reflected radiation from the tissue. The reflected radiation is delivered to a detector (178) along a second transmission path via a circulator (198) which isolates the forward radiation from the second transmission path. The detector has a input which is switchable between the reflected radiation from the second transmission path and a reference signal derived from the forward radiation, wherein detected magnitude and phase information of the reflected radiation to classify the tissue can be compensated for drift in magnitude and phase of the forward radiation by comparison with detected magnitude and phase information of the reference signal.