In this presentation an overview will be given of in vivo magnetic resonance imaging (MRI) and spectroscopy (MRS) techniques for studies in adaptational physiology. Selected examples of MR imaging experiments on marine animals from invertebrates to fishes, as well as on hibernating mammals, will be presented demonstrating the utility of MRI and MRS techniques in evaluating answers in adaptational physiology from the cellular to the whole organismic level. The presentation will contain measurements of organismic behaviour, ventilatory and circulatory performance, cellular energy state and metabolism, ion homeostasis and biophysical state of water/solutes in various organs. Finally, an outlook on recent developments and technical advances that will likely find general applicability in comparative physiological research will be made.