IEC 61300-3-4:2012 describes the various methods available to measure the attenuation of optical components. It is not, however, applicable to dense wavelength division multiplexing (DWDM) components, for which IEC 61300-3-29 should be used. This third edition cancels and replaces the second edition published in 2001. It constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition: - revision of source conditions, launch conditions and power meter parameters; - addition of safety recommendations; - removal of launch condition details for multimode fibres, now referenced in 61300-1. Key words: measure the attenuation of optical components