Establishes requirements for measuring the mode field diameter (MFD) of fibres. Four methods are described: (a) direct far-field scan: (b) variable aperture in the far field: (c) near-field scan: (d) bi-directional backscatter using an optical time domain reflectometer. All four methods apply to type B single-mode fibres, operating near 1310 nm or 1550 nm.The contents of the corrigendum of July 2002 have been included in this copy.