The primary and secondary levels of information which can be derived from the application of X-ray photoelectron spectroscopy (XPS or ESCA) to the study of polymers are discussed. Examples from several applications are presented, both to illustrate points of the ESCA technique and to give a brief indication of some of the current areas of interest. Emphasis is placed on the experimental methods and on conclusions drawn from ESCA data which cannot be obtained by any other analytical technique presently available.