A method for determining whether a probe for measuring the impedance of human or animal body tissue has been placed over a boundary between different tissue types is described. The probe used in the method comprises at least a first (16), a second (22), a third (18) and a fourth (20) electrode arranged such that the third (18) and fourth (20) electrodes are each located substantially the same distance from both the first (16) and second (22) electrodes. The method comprises: driving a current between the first (16) and the second (22) electrodes； measuring a first value of an electrical parameter between the third (18) and the fourth (20) electrode； and determining whether the probe has been placed over a boundary between different tissue types based on the first value. A probe for carrying out the method is also described.
