Abstract: The present invention relates to a non-invasive transdermal apparatus for measuring complex impedance of body tissue. The apparatus comprises: at least two electrodes coupled, in use, to the surface of the skin; a signal source to deliver an electrical potential to the electrodes; a monitor to detect changes in the signal passing through body tissue between the electrodes; and a processor operatively coupled to the signal source and monitor and configured to: use the signal source to apply a potential to the electrodes to reduce the electrical impedance of the stratum corneum; and use the monitor to measure complex transdermal impedance AC signals corresponding to ? and ? dispersion spectra.