Measuring oxygen (O(2)) in tissues has been a central theme of the International Society on Oxygen Transport to Tissue (ISOTT) since its founding 50 years ago in 1973. The initial presentations by many distinguished members reflect this focus and demonstrate the importance of the contributions of the members of ISOTT. This paper considers their work and its legacy in the context of the continuing challenges of making meaningful measurements of O(2) in tissue. Because many …