Publisher Summary This chapter deals with the application of photoelectron spectroscopy in inorganic and organic material systems. Surfaces and interfaces in material systems play an important role in the design of their unique physical and chemical properties. The most effective way to define a surface or interface in a system is to refer to its many degrees of abruptness. A change in any property in a system may be used to define an interface. This chapter illustrates the key principles of XPS or ESCA. The main advantages of ESCA are its simplicity, flexibility in sample handling, and high scientific information content. This chapter discusses several key examples of ESCA in inorganic and organic materials systems. The continued interest in materials science and engineering, biomedical sciences, and surface-related phenomena in general, make it probable that ESCA may remain one of the predominant surface analysis techniques in the coming future.