This chapter introduces hydrogen (1H) magnetic resonance spectroscopy (MRS) of the breast, describes current methods and technical issues, and discusses the clinical applications of 1H MRS for diagnosis of breast cancer and therapeutic monitoring. The spectroscopy component of a breast MR exam is performed immediately after the imaging and dynamic portion of the study, while the patient is in the magnet. Once the lesion morphology and time intensity curves are evaluated, the radiologist can localize a volume of interest (VOI) and begin acquiring MRS data. Currently, breast MRS localization is performed by a technique called single-voxel spectroscopy (SVS). The borders of the voxel, which defines the boundaries from where MRS data are acquired, are delineated using a minimum of three radiofrequency (RF) pulses. Magnetic resonance spectroscopic imaging (MRSI) is performed by using a grid to acquire multiple spectra rather than a single spectrum. The primary advantage of MRSI over SVS is the ability to obtain information about the spatial distribution of metabolites. This can be very helpful in many cases such as those with multiple lesions, heterogeneous lesions, or lesions whose morphologic shape prevent adequate placement of a single voxel. Contrast-enhanced MRI offers nearly 100% sensitivity for detecting breast cancer, yet variable specificity (30-90%). This may be partly because not all malignant breast lesions enhance or have a specific time-signal intensity curve pattern. In vivo 1H MRS is helping expedite testing of experimental chemotherapeutic agents in research laboratories. The use of high-resolution ex vivo 1H spectroscopy enables visualization of the different metabolites as separate peaks in their respective resonance, but with a clinical MR scanner, the resonances from the different compounds are essentially indistinguishable.