This technical report applies to single-mode optical fibres. Its object is to document the methods for measuring the effective area (Aeff) of these fibres. It defines three methods of measuring Aeff. Information common to all the methods is found in the body of this document. Information specific to each method is found in the annexes. The three methods are: a) direct far-field (DFF) b) variable aperture in the far-field (VAMFF) c) near-field (NF) The reference method, used to resolve disputes, is method A, direct far field. Effective area is an optical attribute that is specified for single-mode fibres and used in system designs that may be affected by the non-linear refractive index coefficient, n2. There is agreement in both national and international standards bodies concerning the definition used in this technical report. Methods A, B, and C have been recognised as providing equivalent results, provided that good engineering is used in implementation. The direct far-field is the reference method because it is the most direct method and is named as the reference method for mode field diameter in the ITU. A mapping function is a formula by which the measured results of one attribute are used to predict the value of another attribute on a given fibre. For a given fibre type and design, the mode field diameter (MFD) (IEC 60793-1-45) can be used to predict the effective area with a mapping function. A mapping function is specific to a particular fibre type and design. Mapping functions are generated by doing an experiment in which a sample of fibre is chosen to represent the spectrum of values of MFD and in which the fibres in the sample are measured for both MFD and Aeff. Linear regression can be used to determine the fitting coefficient, k. NOTE Other mathematical models may be used if they are generally more accurate