Although MRI is a routine and invaluable tool in diagnosis and presurgical planning, the related technique of magnetic resonance spectroscopy (MRS) is not often applied. MRS shows the chemical content of brain tissue and can therefore increase the specificity of diagnosis considerably. It also is able to detect abnormality in some tissues which appear normal on conventional MRI, with potential to aid in both discriminating lesion borders and in predicting post-operative outcome. The physical principles of MRS and its technical limitations are discussed, with examples of applications in tumors, epilepsy and traumatic brain injury.