Contrary to the normal behaviour of a macroscopic object that is governed by classic mechanics (e.g. Newton''s Laws), which is familiar to all of us; in the microscopic world, an ''abnormal'' phenomenon called the quantum tunnelling, which is explained by quantum mechanics, plays an important role in some cases (e.g. low temperature). This book demonstrates how quantum tunnelling manifests itself into NMR relaxometry, by taking methyl rotors as a model; As temperature is decreased the continuous transition from classic mechanics behaviour to quantum tunnelling behaviour of methyl rotors is shown. A potential energy source -- the tunnelling reservior of methyl rotors is investigated and the rate of the energy release to lattice is measured. To achieve the experiments a state-of-the-art NMR technique called the field cycling NMR relaxometry is implemented, and this technique is also discussed in this book.