A high frequency, high resolution ultrasonic echo technique is presented for determining the thickness of human skin. An experimental series is described in which the accuracy of this new technique is demonstrated by comparison with a radiological method of proven accuracy. The "ultrasonic biometric ruler" is shown to provide an accurate, simple, noninvasive method for measuring full-thickness human skin. In addition to the determination of skin thickness, it is demonstrated that the underlying subcutaneous fat and muscle can also be noninvasively "explored" with the possibility of identifying a variety of skin and underlying tissue lesions.