Magnetic resonance spectroscopy (MRS) allows non invasive detection of tissue metabolism. Absolute or relative quantification of metabolites and chemical compounds detected by 1H and 31P MRS can today be performed in a number of pathological tissues including different types of cancer lesions, consequently improving accuracy in disease diagnosis and prognosis. Methods allowing quantification of tumour metabolites under in vivo MR spectral profiles have been progressively developed in the past and are now entering the clinical routine. Several methods of quantification have been proposed and validated in healthy tissues in a number of single- and multi-centre studies. Most of these approaches are not always directly applicable to cancer lesions because of their inherent heterogeneity or variability in water content, thus peak area ratios can, in some cases, represent better indicators of disease progression and response to therapy. An additional important limitation may derive from the excessive duration of some MRS measurement protocols, which would be added to a routine clinical MRI examination in the same session. We propose a critical overview of the principal methods currently applied for the quantification of metabolites and chemical compounds, mostly detected by 1H MRS in tumours.