Standard magnetic resonance imaging (MRI) is a prominent clinical imaging modality used to diagnose and study diseases in vivo. It is principally based on the detection of the nuclei of hydrogen atoms (the proton; symbol 1H) in water molecules in tissues. X-nuclei MRI (also called nonproton MRI) is based on the detection of the nuclei of other atoms (X-nuclei) in the body, such as sodium (23Na), phosphorus (31P), chlorine (35Cl), potassium (39K), deuterium (2H), oxygen (17O), lithium (7Li), and fluorine (19F), that can also be detected using modified software and hardware. X-nuclei MRI can provide fundamental, new metabolic information related to cellular energetic metabolism and ion homeostasis in tissues that cannot be assessed using standard hydrogen MRI. This book is an introduction to the techniques and biomedical applications of X-nuclei MRI.