In situ X-ray absorption spectroscopy is a powerful emerging technique that has the capability to observe the changes in ongoing electrochemical reactions. It is already well established in materials science, and it is becoming a significant tool for the electrochemical community. As with all X-ray absorption spectroscopies, extended X-ray absorption fine structure (EXAFS) has the advantage of being element specific. Interpretation of the spectra at different states of charge can provide very useful quantitative and qualitative information about the valence change of the constituent elements in the cathode material during the ongoing electrochemical reaction, the degree of distortion or changes in structure from the initial state of charge to the final state of charge and provide valuable information about the extent of degradation of the cathode material during continuous cycling. It can also provide valuable insight about how the nature of the electrochemical reactions changes when one of the transition metal constituents is removed or increased in content in the cathode material. It is often important to adjust the composition of the cathode material in order to achieve high specific capacity and long-term stability in Li-ion cells. This article details the development of the in situ XAS techniques to study electrochemical reactions using various X-ray absorption spectroscopies which are now possible with the advent of third generation synchrotron radiation sources and improved end stations. The strength of in situ EXAFS techniques is illustrated using examples of various interesting transition metal oxides. In this way, we aim to encourage chemists, chemical engineers and materials scientists to consider in situ X-ray absorption spectroscopy as an effective tool for developing an understanding the electronic structure of materials and the changes that it undergoes during electrochemical reactions. (c) 2006 Elsevier B.V. All rights reserved.