The feature that is perhaps the most characteristic of an XPS spectrum, i.e. the step in background which accompanies every peak, is also its most enigmatic. The simple interpretation given in early texts was that the step is created by extrinsic losses as the electron travels through the material has not survived the work of Tougaard@footnote 1@ who has shown that the extrinsic losses build up rather slowly on the low kinetic energy side of the peak. In fact when the Tougaard function is subtracted from a typical peak there remains a peak with a considerable tail. Interpretation of this tail in a quantitative manner has always posed problems, mainly because the standards used for sensitivity factors do not include the tail and in most cases of analysis the tail extends beyond the window used for the narrow scan. Thus to undertake quantification in XPS analysis it is normal to remove a background which is based on the original Shirley algorithm,@footnote 2@ i.e. an integration of the peak lying above background using an integration constant which allows the background to merge with the experimental data at some point close to the peak. A browse through a collection of standard xp spectra of the elements quickly reveals what many of us recognise from experience: That the intensity removed by the Shirley background differs from one element to another. Over the past few years, working with Pr.A.M.Salvi, we have shown that this part of the background structure varies in a systematic manner across a row of the periodic table. In this review we bring together results published in several journals@footnote 2-6@ in an attempt to give a unified account of the progress so far made. It will be shown that the intensity of the background can be distinguished from the Tougaard, extrinsic loss, background and characterised by a single parameter. This 'shape' parameter can be of value in peak fitting, especially when fitting over lapping peaks as occurs with oxide films on metals. There is also an element of chemical state information contained in the peak itself. For example we have shown that when aluminium participates in the formation of an aluminide with one of the 3d transition metals, then the aluminium gains the background imprint typical of the 3d elements. Similar findings occur in the formation of other covalent compounds and in the chemisorption of molecules to transition metal substrates. In conclusion the review will show that the Shirley background, far from being part of the spectrum to be discarded, actually contains information of a secondary nature which can be useful in interpretation of the primary analytical results. @FootnoteText@@footnote 1@S. Tougaard, Applied Surface Science, 100/101, pp 1-10 (1996) @footnote 2@D.A. Shirley, Phys.Rev.B, 5 4709 (1972) @footnote 3@Anna Maria Salvi and James E Castle, J. Elec Spec & Related Phenomena, 94 pp 73-88 (1998) @footnote 4@Anna Maria Salvi and James E Castle, "The intrinsic asymmetry of Photoelectron Peaks: Dependence on Chemical State and Role in Curve Fitting", J. Elec Spec & Related Phenomena, 95 pp 45-56 (1998) @footnote 5@J.E.Castle, S.J.Greaves, M.R. Guascito, and A.M.Salvi, "A New Probe of Bonding States in Intermetallic Compounds" Phil Mag. 79, pp 1109-1129 (1999). @footnote 6@J.E.Castle, A.M.Salvi*, M.R. Guascito, "A Substrate-Related Feature in the Loss Structure of Contamination-C1s" Surface and Interface Analysis, 27, 753 - 760 (1999) @footnote 7@J.E.Castle, H.Chapman-Kpodo, A.Proctor** and A.M.Salvi*"Curve-Fitting in XPS Using Extrinsic and Intrinsic Background Structure" J.Elec.Spec.and Rel Phenomena, 106 pp 65-80 (1999)