IEC 61291-1:2018 applies to all commercially available optical amplifiers (OAs) and optically amplified assemblies. It applies to OAs using optically pumped fibres (OFAs based either on rare-earth doped fibres or on the Raman effect), semiconductors (SOAs), and waveguides (POWAs). The object of this document is - to establish uniform requirements for transmission, operation, reliability and environmental properties of OAs, and - to provide assistance to the purchaser in the selection of consistently high-quality OA products for his particular applications. Parameters specified for OAs are those characterizing the transmission, operation, reliability and environmental properties of the OA seen as a "black box" from a general point of view. In the sectional and detail specifications a subset of these parameters will be specified according to the type and application of the particular OA device or assembly. This fourth edition cancels and replaces the third edition published in 2012. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition: - terms have been added for parameters from IEC 61290-4-3 and IEC 61290-10-5; - Clause 4 Classification has been removed, since this system is judged to be unused; - the definition of polarization mode dispersion (PMD) has been simplified. Keywords: optical amplifiers (OAs)