Publisher Summary This chapter discusses some experimental data related to the adenosine triphosphate (ATP) in cells and organs. The chapter reviews two sophisticated 31P-nuclear magnetic resonance (NMR) techniques: saturation transfer and inversion transfer. These techniques are designed to measure the chemical exchange of phosphorus nuclei among the major phosphate-containing metabolites in cells. The majority of techniques for studying metabolic changes at the cellular and organ levels require an eventual destruction of the tissue and a subsequent analysis of the extract. These techniques demonstrate that not only the energetic state of cells and their components are possible to assay but also the modification or manipulation of metabolic defects at the cellular level is feasible. The chapter also reviews data concerning the measurements of ATP at the cellular level.