AbstractA discussion is presented of the evaluation of multiple relaxation components from water protons in biological tissue. The principal focus is to draw attention to the way in which limitations in the raw NMR data, such as signal‐to‐noise ratio, data sampling density and acquisition window width, affect the precision and resolution in the processed multiple component solution of the return to thermal equilibrium. The second issue discussed is the interpretation of these multiple components in terms of microstructural compartments of the biological sample and, thirdly, we outline some of the successes in determining regional and pathological variations in microstructure in the human body in‐vivo, using the technique of multiple relaxation components. Copyright © 2001 John Wiley & Sons, Ltd. Abbreviations used: CNS central nervous system CPMG Carr–Purcell–Meiboom–Gill sequence RBC red blood cells NMR nuclear magnetic resonance NNLS non‐negative least squares PNS peripheral nervous system SNR signal‐to‐noise ratio TE echo time.