The aim has been to describe the general principles in the use of ultrasonics for localization, and to compare ultrasonic sections with X-ray sections; also to try and stress the completely different principles involved in using the ultrasonic echo method and X rays. An attempt has been made to make it clear that ultrasonics is not of use for obtaining better cross-sections than those provided by X rays, but for locating interfaces which would not be detected by X rays at all. The 1 or 2 examples given may perhaps show the way to other instances in which ultrasonics may be of use where X rays are not. Ultrasonics could be a very useful general diagnostic tool, taking its place beside other techniques and filling gaps which X-ray techniques perhaps cannot fill.