X-ray absorption spectroscopy has played vital roles in developing our understanding of the molybdenum and tungsten enzymes. This chapter discusses the technique of X-ray absorption spectroscopy, with an emphasis on experimental and data analysis techniques, and on strengths and weaknesses of the method. The role that X-ray absorption spectroscopy has played in the state of our knowledge of the molybdenum and tungsten enzymes will then be reviewed, with selected examples chosen to illustrate the strengths and weaknesses of the method.