This paper describes a novel attempt to determine quantitatively the area of contact between stationary and sliding contacts by means of an ultrasonic technique. The basic idea is to generate an ultrasonic wave in one body and measure the impulse passing into the second body through the interface. Analysis shows that the transmitted pulse depends on the elastic stiffness of the interface. According to theory the stiffness of a single circular contact is proportional to the diameter of the contact and not to its area and this has been confirmed by simple scaled-up experiments. Because of this the stiffness of multiple contacts is not determined unequivocally by the true area of contact. The scaled-up experiments show that for widely separated contacts the stiffness is the sum of the stiffnesses of the individual contacts; if they are close together it is determined primarily by the size of the embracing circle of contact. Only when the apparent area of contact is more than ten times the true area does the effect of the empty spaces on the stiffness become appreciable. The ultrasonic experiments confirm all these conclusions and the technique has been applied to the static contact between metals, between smooth and rough glass, between rubber and between polymers. In some cases the results are able to distinguish between elastic and plastic deformation in the contact zone. The technique has been extended to sliding contacts and the results show that with ductile materials junction-growth may occur. Although the ultrasonic method suffers from the major disadvantage in that it cannot unequivocally distinguish between the true and the apparent area of contact it has the great advantage that, in contrast to electrical resistance measurements, it can be used for nonmetals and is scarcely affected by the presence of oxide films. Again in contrast to optical methods, it can be used with opaque materials.