Heretofore observations of minute surface changes in tooth structures have been made with the aid of several methods of varying accuracy. Visual inspection is the most primitive and inaccurate, but nevertheless most often used technic. It must be recognized that this method has many limitations and therefore is applicable, at best, only for preliminary study. Linear, weight, or volume measurements of changes on tooth surfaces are better methods but here again many difficulties are evident. For instance, since surface changes after application of drugs or other chemicals are usually slight, measurements of dimensional changes or weight losses or gains lack accuracy. Then too, delicate allocation of changes to specific surface portions is almost impossible. Photographic reproduction using reflected light is probably the best method thus far employed. Even with this fine technic certain difficulties present themselves. When we deal with a very smooth or a very dull surface, pictures obtained are not very instructive. It is difficult to observe small changes in the surface. In addition to this, intensity and angle of illumination are rather critical. In many instances, apparent extreme variations can be shown in the same surface merely by changing the angle of the incident light. Another technic for observing changes in relatively thick specimens is frequently used for histologic studies. Thin ground sections are made and observed either by transillumination, or by reflected light as mentioned above. This technic is admirable when only a few sections of a given piece of material are desired. The number of serial sections that can be made from any given sample is obviously limited. In 1940 a technic was introduced2 which consists essentially of making a micro-impression of a surface in a transparent plastic, and viewing this impression under magnification. A similar technic, known as the Fax-film3 has been suggested for the study of metallographic surfaces. The Fax-film technic is accepted by the Bureau of Standards as a method of comparing roughness of surfaces. An application of this micro-impression technic has been adapted to the study of tooth surfaces. A description of the essentials of this method follows.