X-ray photoelectron spectroscopy (XPS or ESCA) has reached a state of maturity in which some of its common uses may be considered routine. There is a danger, however, in the possible over-interpretation of this status. (Consider, for example, the hundreds of incorrect ESCA analyses in high Tcsuperconductivity.) Thus, although recent advances in instrumentation have simplified general operations, they have also clearly identified a variety of previously undetected or misunderstood features in the technique that seem to suggest the potential of a more powerful analysis tool. These new areas require a much more sophisticated understanding of the photoelectron process and its potential uses. In the present article we provide a description of the background, present status, and possible future uses of some of these features of photoemission spectroscopy, including: (1) the charging shift and Fermi edge referencing, (2) valence band analysis, (3) XPS induced loss spectroscopy, (4) surface-to-bulk chemical shifts, (5) small cluster analysis, (6) photoelectron microscopy, (7) inverse photoemission, (8) resonance photoemission, (9) photoemission of adsorbed xenon, (10) photoelectron diffraction, and (11) liquid phase photoemission. (Some prejudicial discretion has been exercised in the degree of emphasis on each of these topics.) Novel analyses are described of various oxides (including zeolites and high Tcsuperconductors), inert hydrocarbon polymers, carbon filled metal ceramics, supported metals catalysts, unique structures, and a variety of other systems. This review is divided into two parts. The first part treats the developmental and theoretical background of ESCA in detail, including established ESCA procedures (up to recent studies of the causes and uses of loss spectroscopy in ESCA), laying the basis for the description and elaboration of promising new directions in ESCA given in part two of this review. This two-part work is intended to serve as a useful treatise defining the underlying characteristics and broad capabilities of the ESCA technique.