Magnetic resonance spectroscopy (MRS) offers almost unique noninvasive access to cellular biochemistry and metabolism in vivo. The manifold clinical uses of MR imaging have driven extraordinary technical innovation and application. Developments in MRS have been rather slower, until the recent advent of dynamic nuclear polarisation and high-field clinical scanners. As well as this methodological innovation and implementation, full exploitation of the potential of MRS in clinical research and practice will requires a clear quantitative understanding of the nature of the measurements and their relationship to the dynamic physiological processes that they probe. In this lecture I will discuss some of these key issues in some clinical-research applications of MRS in skeletal muscle, liver, brain and heart.