In this chapter, the use of Pt-porphyrin-based extracellular and intracellular O2 sensing probes (ecO2 and icO2) on a TR-F plate reader format is described and critically assessed. The principles underpinning extracellular measurement are outlined and the assessment of biological oxygen consumption in prokaryotic cells, eukaryotic cells and small organisms is outlined along with a description of how such measurements can contribute to the development of a detailed picture of cell metabolism. The use of a mathematical model describing the distribution of local O2 gradients within biological samples is also described. Finally, the principles of icO2 sensing are discussed and some short case studies are provided for demonstrating the utility of icO2 probes for the monitoring of cellular function and metabolic perturbations and how such measurements can be allied to other bioenergetic markers to generate a more complete picture of metabolic status