Magnetic resonance imaging (MRI) infused Radiology with rejuvenating vigor in the 1980s, owing credit to a couple of magnetic resonance spectroscopy (MRS) experiments performed in 1973. MRI has since been embraced by the radiology and medical communities. If the goal of MRS is to measure many chemicals in a homogeneous magnetic field, then the function of MRI is, in general, to measure one chemical - water - in an artificially created inhomogeneous field. Combining spectroscopy principles with technologies developed over the past two decades for MRI presented the philosophical appeal of non-invasively measuring metabolic molecules in living tissue, and led to the explosive developments in the last decade of in vivo MRS, and more recently MRSI, in the settings of diagnostic radiology. This review is intended to discuss the basic technologies of the current trends in the field of in vivo MRS and MRSI, especially the inherent predilections of individual techniques to the study of certain disease states. Following a historical introduction, individual techniques and their clinical applications, found in publications between January 2000 and October 2004, are reviewed in connection with related ex vivo results, after which the practical aspects of in vivo MRS and MRSI in clinical settings are discussed.