Among the vast number of metabolites in living tissues, metabolites detectable by in vivo MR spectroscopy are limited to those present in high concentrations, and the actual number is only 10 to 20. None is disease-specific. Interpretation of MRS data, therefore, must be based on general knowledge of biochemical processes in association with pathological changes. Each spectrum is a window on the actual biochemical changes taking place within the living tissues, but the reality entails a wide and confusing variance. Continuous expansion of the knowledge may reduce the uncertainty of interpreting MRS data.