Molecular oxygen has multiple roles in cells, which can have different dependencies on the amount of oxygen at particular sites. Therefore, in order to relate oxygen to the processes under consideration, one must know the amount of oxygen present at the site of interest. The use of the various techniques generally is complementary rather than competitive. The key to their successful use requires an understanding of the capabilities of the techniques and of the nature of the process that is being investigated. This paper provides a framework for obtaining such an understanding.