Establishes uniform requirements for measuring the attenuation of optical fibres. Four methods are described: (a) cut-back, (b) insertion loss, (d) backscattering, (d) modelling spectral attenuation. Methods (a), (b) and (c) apply to all categories of class A multimode fibres and class B single-mode fibres. Method (c), backscattering, also covers the location, losses and characterization of point discontinuities. Method (d) has been demonstrated only on class B fibres.